Preheat oven to 425 degrees.: You will notice a warm, dry smell as the oven comes up to temperature, which helps the quiche begin with an immediate blast of heat for a good initial rise. Preheating matters because a hot oven encourages the custard to puff, setting the outer edges while the center finishes slowly. A common mistake is placing the dish in a cold oven, which can yield a dense texture, so wait until it reaches the full 425 degrees before baking.
In a large skillet on medium-low heat, add oil and sauté onion and garlic until tender, about 4 minutes. Add diced bell pepper, zucchini, broccoli, and sun-dried tomatoes and continue sautéing for 2 minutes.: As the olive oil warms, it will shimmer slightly, releasing a faint fruity aroma; when you add the onion , you'll hear gentle sizzling and smell the sweetness emerging as the pieces soften. The garlic becomes fragrant almost immediately, so keep the heat moderate to avoid charring, which smells sharp and bitter. Sautéing slowly builds a sweet, savory base that the custard layer depends on, and rushing this step often leaves raw onion or acrid garlic notes in the final dish.
In a medium mixing bowl, whisk together eggs, egg whites, milk, spices, and 1/4 cup parmesan cheese. Lightly spray a 9 inch pie dish, add sautéed vegetables. Pour egg mixture over vegetables, make sure to cover all veggies.: When you add the red bell pepper , green bell pepper , zucchini , broccoli , and sun-dried tomatoes , the pan will fill with mixed vegetal aromas and a brighter sweetness. Two minutes lets the vegetables warm through and release a little steam, but keeps them slightly crisp so they hold textural contrast against the creamy custard. Overcooking here can soften them too much and cause excess moisture, which may lead to a soggy center in the finished quiche.
Loosely cover with foil and bake 10 minutes at 425 degrees, reduce heat to 350 and continue baking 20-25 minutes. Remove foil the last few minutes of baking time and sprinkle with the remaining parmesan cheese. Quiche is done when it puffs and a knife inserted in the center comes out clean.: As you whisk the eggs , you'll see the color become even and pale, and small frothy bubbles will form at the surface indicating air has been incorporated for a light set. The addition of milk creates a silkier mouthfeel, while the parmesan cheese begins to dissolve into the mixture, lending savory depth. Whisking thoroughly ensures an even texture; insufficient mixing can leave streaks of dense egg in the bake, so whisk until uniform.
Lightly spray a 9 inch pie dish, add sautéed vegetables.: When you transfer the vegetables into the prepared pie dish, press them into an even layer so every slice will get a good distribution. The pan should feel warm to the touch but not hot enough to steam the egg mixture on contact. Using a light spray prevents sticking and aids in clean slicing later. If the vegetables mound unevenly, the quiche may set unevenly, so spread them out evenly for consistent baking.
Pour egg mixture over vegetables, make sure to cover all veggies.: As you pour the custard, watch it flow into crevices and around florets, smoothing to form a glossy surface. The visual cue that the liquid has filled the dish uniformly is a continuous reflective layer with no dry spots, which ensures even setting. If you pour too quickly and create bubbles, sit the dish for a minute so small bubbles rise and pop, preventing holes in the finished surface.
Loosely cover with foil and bake 10 minutes at 425 degrees, reduce heat to 350 and continue baking 20 to 25 minutes.: The initial high heat will make the custard puff, visible as it rises and firms at the edges; you may hear a faint settling sound as steam escapes. Lowering to 350 degrees finishes the interior gently so it sets without cracking. A common pitfall is leaving the foil too tight, which traps steam and makes the top soggy; a loose tent maintains heat while allowing moisture to escape.
Remove foil the last few minutes of baking time and sprinkle with the remaining parmesan cheese.: Taking off the foil exposes the top to dry heat so the surface can take on a light golden color and the parmesan cheese can melt and slightly brown, adding a savory crust. You will see tiny bubbles subside and a gentle browning appear as this happens. If you skip this step, the top may remain pale and lack that toasty flavor, so plan to uncover briefly for color.
Quiche is done when it puffs and a knife inserted in the center comes out clean.: Check doneness by observing a gentle jiggle in the center that disappears as it cools, and confirm with a knife that comes out mostly clean, perhaps with a hint of moistness but no raw batter. The aroma should be warm, eggy, and vegetal, with toasted cheese notes on top. Overbaking will dry out the custard, so remove it promptly when the knife test passes to preserve a tender interior.