Preheat your oven to 400 ℉.: You should feel the warm hum of the oven as it comes to temperature, and preheating ensures even cooking and a set custard. The hot oven encourages the edges to rise slightly and develop a delicate browning, which creates texture contrast against the tender center. A common mistake is skipping preheating, which can lead to uneven set and a runny middle, so always wait until the oven indicates it is fully heated.
In a bowl, whisk together eggs, almond milk, minced garlic, sea salt, and black pepper until well combined. Set the mixture aside.: When you whisk, listen for the gentle froth forming on the surface and watch the color even out to a uniform golden hue, which signals thorough mixing. This aeration helps the final custard be light but still creamy. If you under-whisk, pockets of uneven texture may form; over-whisking is less of an issue here but try to stop when the mixture is smooth and slightly airy.
Add the broccoli, onions and feta to the egg base.: The mixture should smell faintly garlicky and look dotted with green and white, which means the flavors will be distributed. Folding in the broccoli and onion rather than stirring vigorously helps preserve small pockets of air and avoids crushing the florets. A common error is adding too-large broccoli pieces, which can cause uneven cooking, so aim for small florets so they cook through in the oven.
Transfer the mixture to a skillet greased with olive oil. Bake in the preheated oven for 15 to 20 minutes, or until the eggs are set.: As the frittata bakes, notice the edges starting to puff and the surface turning matte rather than shiny, which means the eggs are firming. You may hear a faint sizzle as the oil crisps the perimeter. The visual cue to look for is a uniformly set center with no wobble when you gently jiggle the pan. Avoid opening the oven frequently, because temperature drops can add to your cook time and lead to uneven set.
Remove the frittata from the oven, let it cool slightly, season to taste, and serve.: Cooling allows the custard to finish setting and makes slicing easier. You will smell a warm amalgam of roasted onion and baked broccoli , and the top should look lightly golden in spots. If you slice too soon while it is piping hot, the wedges may fall apart, so give it a short rest. A typical mistake is over seasoning before baking; wait to taste until it rests because the flavors concentrate slightly as it cools.
DEVOUR!: The first bite should be warm, with a creamy interior contrasted by the tender broccoli and the playful saltiness of feta cheese . Pay attention to texture and temperature as you eat, and notice how the olive oil imparts a gentle richness. Resist the urge to reheat aggressively, as high heat can dry out the custard; gentle reheating preserves the ideal mouthfeel.