Onion Frittata: Forty Minutes of Onions, Ten of Eggs
A frittata is a ratio problem in disguise. Most versions are too eggy because the cook treats it as an omelette with fillings. This one is built the other way round — 700 g of onions to 10 eggs — so the eggs are a binder rather than the main event, and the slice holds its shape at room temperature on a buffet table. The onions take forty minutes and you can walk away for most of them.
Ingredient card
| Item | Amount |
|---|---|
| Yellow onions, sliced 4 mm | 700 g, about 4 large |
| Olive oil | 80 ml |
| Unsalted butter | 20 g |
| Fine salt | 1.5 tsp |
| Fresh thyme, leaves picked | 1 tbsp |
| Large eggs | 10 |
| Double cream | 80 ml |
| Parmesan, finely grated | 60 g |
| Black pepper | 1/2 tsp |
Scaling
for 2: Half the eggs and half the onions in a 20 cm oven-safe pan. Bake 14 minutes.
for 4: As written: 10 eggs, 700 g onions, 26 cm pan. This is the batch the timings assume.
for 8: Double the onions and use two 26 cm pans rather than one 30 cm pan. A single thick frittata needs 35 minutes in the oven and the bottom sets rubbery before the centre is done.
Time budget and burner map
Make ahead
- Caramelise the onions up to four days ahead and refrigerate. This is the whole job; the rest is ten minutes. Reheat them in the pan before adding the eggs.
- A frittata is better warm than hot, and it is genuinely good cold. Bake it two hours before service and leave it on a board under a cloth.
- Grate the parmesan and pick the thyme the night before. Both keep in the fridge in small containers.
To the pass
- Turn out onto a board, not a plate, and cut into wedges with a long knife rather than a sawing motion.
- Top with a handful of dressed bitter leaves — rocket or chicory — which cuts the richness.
- A squeeze of lemon over the leaves at the last second, never over the frittata.
Where it goes wrong
- Rushing the onions. Under 30 minutes they are sweet but thin; the deep, jammy flavour only arrives in the last ten. If they are sticking, add a tablespoon of water, not more oil.
- Too hot an oven. Above 190 °C the eggs puff, brown and then collapse as they cool, leaving a dry, spongy slab. At 175 °C the custard sets slowly and stays tender.
- Overcooking the centre. Take it out when the middle still wobbles like a set jelly — it continues to cook in the pan for another five minutes. A thermometer in the centre should read 82 °C.
The method, in full
Forty minutes of onions is the recipe
Onions start at about 89 percent water. Sweating them with the lid ajar for 34 minutes drives most of that off without browning, and what is left is concentrated allium sugars. The last 6 minutes at higher heat is when the Maillard reaction happens and the colour goes from pale gold to deep amber. Cut it short and you have a bland frittata no amount of parmesan fixes.
The salt goes in at the start, not the end. Early salt draws water out of the cell walls so it can evaporate, and it seasons through rather than sitting on the surface.
The egg-to-onion ratio and why it matters
Ten eggs to 700 g of onions is roughly a 60/40 split by volume after cooking, which is the point at which the slice holds together when cut but still tastes of onion. Most home frittatas run closer to 80/20 and taste like a baked omelette.
The cream and parmesan are structural, not decorative. Cream dilutes the egg proteins so they set more slowly and stay tender; parmesan adds salt and a small amount of starch that helps the custard hold. Skip the cream and you need to drop the oven to 160 °C and add four minutes.
Hob then oven, and why you need both
The four minutes on the hob sets the base and the rim, which gives the frittata a clean edge and stops it sticking. The oven does the rest evenly. If you try to do it all on the hob the bottom scorches; all in the oven and the base never sets properly and the slice weeps when you cut it.
Use a pan that is oven safe to 220 °C and has a metal handle you can put under a grill. If your handle is plastic, wrap it in two layers of foil — but a frittata that needs the grill is a frittata that was too thick to begin with.
Kitchen questions
Can I make a frittata with egg whites only?
Yes, but replace each whole egg with two whites plus a teaspoon of olive oil, and reduce the oven to 160 °C. Whites set firmer and faster, so the window between done and rubbery is about two minutes.
Why did my frittata weep water when I cut it?
Almost always undercooked eggs plus wet vegetables. If you add courgette, tomato or spinach, salt and drain them for 20 minutes first. The onion here is dry by the time the eggs go in, which is exactly why it does not weep.
How long can it sit out?
Two hours at room temperature is fine; beyond that refrigerate it. It keeps four days covered and is good cold, which makes it a legitimate make-ahead lunch rather than just a brunch item. This is general kitchen guidance rather than food-safety advice for any particular situation.
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