Why Onions Are the Backbone of Plant-Based Flavour
💥 Part of our “Five Pungent Spices” series exploring the bold, aromatic roots of plant-based flavour. See also: Garlic, Asafoetida, Leeks and Chives.
🌱 Introduction
From curries and stir-fries to stews and salads, onions are everywhere. But they’re not just background noise. Onions provide the bedrock of savoury, sweet, and umami depth in plant-based cooking, transforming dishes from bland to brilliant. Let’s unpack how.
🧪 The Chemistry of Onion Flavour
🥲 Why Onions Make You Cry
Cutting an onion breaks its cells, allowing the enzyme alliinase to react with sulphur-containing compounds (specifically S-alk(en)yl cysteine sulfoxides). This forms sulfenic acids, which are unstable, and one, 1-propenyl sulfenic acid, is converted by lachrymatory factor synthase (LFS) into syn-propanethial-S-oxide. That’s the gas that makes you tear up.
🧄 Pungency and Depth
While the tear-inducing gas quickly dissipates, other sulfur volatiles like thiosulfinates, disulfides, and trisulfides remain. These compounds give raw onions their bite and contribute to flavour complexity. As onions cook, many of these harsh compounds degrade or transform into milder, sweeter, and often umami-enhancing molecules, including meat-like thiopropanal S-oxide derivatives (MMPs).
🔥 The Magic of Heat: Maillard vs. Caramelisation
🌡️ Caramelisation is the browning of sugars. In onions, natural sugars like fructose and glucose undergo pyrolysis at high heat (~160°C+), forming sweet, nutty, and toasted notes.
🍳 Maillard Reaction is the browning between amino acids and sugars, also triggered by heat (~140–165°C). This creates deeper, savoury, roasted flavours — critical to the "meaty" complexity of well-browned onions.
🧠 Key Insight: Most browning in onions (especially in sautés and caramelisation) is Maillard-driven, not just sugar caramelisation. Slow cooking maximises both, layering sweetness and umami.
🍽️ Cooking Techniques for Maximum Flavour
🥄 Sautéing
Start onions in a cold pan with oil and bring up to medium-low heat. Let them sweat (covered if needed) until translucent. Then uncover and stir occasionally, adjusting heat as needed to avoid burning. Perfect for base flavour in soups, dals, and sauces.
⏳ Caramelising
True caramelisation takes time — about 45–60 minutes. Use a wide, heavy-bottomed pan over low heat. Stir occasionally, deglazing the pan with water, wine, or stock to lift flavourful browned bits (fond). Adding a small pinch of salt early helps draw out moisture. A teaspoon of sugar or balsamic vinegar can enhance browning.
🧊 Taming Raw Bite
To mellow raw onions (e.g. for salads or sandwiches), soak thin slices in cold water or vinegar for 10–30 minutes. This leaches sulfur volatiles, softening their sharpness.
🧅 Choosing the Right Onion
🔸 Yellow/Brown – All-purpose, best for sautés, soups, and stews. Balanced pungency and sugar content.
🔸 Red – Mild and slightly sweet. Ideal raw (salads, pickles), though colour fades when cooked.
🔸 White – Sharp and crisp. Common in Mexican salsas and chutneys.
🔸 Sweet (e.g. Vidalia) – Low sulphur, high sugar. Great raw or roasted, but less shelf-stable.
🔸 Shallots – Mild, garlicky. Excellent for vinaigrettes, sauces, or slow caramelisation.
🔸 Scallions/Spring Onions – Mild green tops, great raw or stir-fried.
🌏 Global Uses – From Mirepoix to Mujadara
Onions are a cornerstone in global culinary traditions:
🔸 Mirepoix (France): Onion, celery, and carrot base for soups and sauces
🔸 Sofrito (Latin America/Spain): A regional mix of sautéed onion, garlic, peppers, and herbs—used to flavour beans, rice, and stews
🔸 Masala base (India): Browning onions with garlic, ginger, and spices to build rich curry foundations
🔸 Pho (Vietnam) – Charred onions and ginger deepen the broth aroma
🔸 Mujadara (Middle East): Lentils and rice cooked with caramelised onions, plus crispy onion garnish
These techniques build layered, rich, savoury bases – essential in plant-based meals
🥗 Nutrition & Digestibility
✅ Onions are:
A source of vitamin C, quercetin (an antioxidant flavonoid), and fructans (inulin and FOS – prebiotic fibres)
Low in calories, mostly water (~89%)
⚠️ Caution:
High-FODMAP: Fructans can cause bloating in sensitive individuals (e.g. IBS).
Allergies are rare but can cause contact dermatitis or digestive upset.
Cooking (especially boiling) reduces FODMAP content; spring onion tops (green part) are a lower-FODMAP option.
📦 Storage Tips
Whole onions: Keep cool, dry, and well-ventilated – not in the fridge.
Keep away from potatoes, which emit moisture and gas that spoil onions.
Cut onions: Store in an airtight container in the fridge; use within 7–10 days.
💡 Serving Ideas
Blend caramelised onions with silken tofu for a creamy dip
Add to lentil soup or miso broth for umami depth
Use quick-pickled red onions to brighten grain bowls and tacos
Stir sautéed onions into tofu scrambles, curries, and pilafs
Grill thick onion rings as a burger topping or BBQ side
🧈 Pro Tip: Blend silken tofu with sautéed or roasted onions for a rich, dairy-free base sauce.
🌟 Final Takeaway
Onions are not just a seasoning — they’re a structural element. Their transformation through cutting, heat, and time makes them a cornerstone of flavour in global plant-based cuisine. Understand them, cook them well, and they’ll reward you with unmatched depth.
Let onions be your flavour architect.