Pour the onion-egg mixture into the prepared glass baking dish, smoothing the top with a spatula. You should see ribbons of onion marbled through the pale yellow base.
Bake in the preheated oven for 25–35 minutes, or until the egg bake is puffed, set in the center, and the top is a soft golden color with some deeper brown onion bits visible. A knife inserted in the middle should come out mostly clean, without runny egg.
Remove the dish from the oven and let it rest for at least 5–10 minutes. The bake will deflate slightly as it settles, becoming tender and sliceable.
Cut into thick squares and serve warm, enjoying the fluffy, custardy texture and the sweet, savory flavor of the caramelized onions.
For a slightly richer version, you can replace 1–2 of the eggs with 1/4 cup of heavy cream or full-fat yogurt, but note this adds an extra ingredient beyond the base three. To keep it strictly three-ingredient plus fat, focus on technique: caramelize the onions deeply for more sweetness and complexity, and whisk the eggs thoroughly with a bit of extra air for maximum fluff. If you prefer a firmer, more sliceable texture (closer to a dense frittata), bake a few minutes longer until the center is completely set. For a softer, custard-like texture, pull it from the oven as soon as the middle no longer looks wet and allow the carryover heat to finish the cooking. You can also change the character of the dish by using red onions for a slightly sharper, more aromatic flavor or sweet onions for a very mellow, dessert-like sweetness. Food safety tips: Always cook the onions until they are fully softened and hot throughout; avoid tasting from the pan with the same utensil you’ll use in the egg mixture unless it’s cooled and clean. Make sure the bake reaches a safe internal temperature (160°F/71°C) in the center so the eggs are fully cooked. Cool leftovers promptly, within 2 hours, and refrigerate in a covered container for up to 3 days. Reheat individual slices gently in the microwave or a low oven until steaming hot, and discard if there are any off smells or signs of spoilage.
