Preheat your oven to 350 degrees Fahrenheit and lightly coat a 6-cup muffin tin with nonstick spray. Divide half of the green asparagus, bell pepper, and tomatoes among the cups in the muffin tin.: You should feel the warmth in the oven air when you open it, and the preheated environment helps the eggs begin setting immediately, creating a gentle rise and even texture. The sound of the oven clicking off is your cue that it reached temperature, and a light spray prevents sticking while allowing the muffins to brown slightly at the edges. If you skip preheating the muffins may cook unevenly and take longer, which can yield a rubbery center. A common mistake is using too much spray, which can pool in the cups and cause the bottoms to fry instead of bake, so use a thin, even coating.
In a large bowl add the eggs, chopped parsley, and season with a pinch of salt, pepper, and cayenne pepper. Whisk gently until combined.: As you place the vegetables into each cup you should see a colorful base that will be partially covered by the egg, giving texture and pockets of flavor. Spread the pieces so each cup gets an even distribution, and press any tall bits down slightly so they do not stick out above the rim. This ensures uniform cooking and prevents exposed veg from charring. A frequent misstep is piling one cup with too much veg and leaving another nearly empty, which results in uneven muffins; portion carefully with a teaspoon or small measuring cup for consistency.
Fill each muffin cup with the egg mixture, top with the other half of the vegetables, and sprinkle the feta evenly over the top.: When you crack and whisk the eggs you should see them become slightly frothy and smooth, with ribbons forming as the whisk moves through. The aroma of fresh parsley will lift as you stir, and the seasoning begins to build depth. Whisking adequately incorporates air for a lighter texture, but over whisking can make the custard too airy and cause holes in the finished muffin, so aim for uniformity without excessive foam. A common mistake is not seasoning the egg base enough, which leads to a bland result, so taste a small spoonful of beaten egg before baking if you are comfortable doing so.
Bake the muffins for 20-25 minutes in the preheated oven until the eggs are set. Remove the baked egg muffins from the oven and let cool for a few minutes. Loosen the muffins with a knife, then remove them from the pan and let them cool slightly on a wire rack.: You want a homogeneous mixture where the yolks and whites are fully integrated, and the herbs and spices are evenly distributed. The surface should look cohesive and free of streaks, with small bubbles forming if you whisk briskly. Proper whisking ensures even cooking and consistent flavor in every bite. Avoid leaving lumps of parsley or unbroken yolk; both lead to inconsistent pockets of flavor or texture in the baked muffins.
Fill each muffin cup with the egg mixture, top with the other half of the vegetables, and sprinkle the feta evenly over the top: Pour carefully so each cup receives an equal amount of the egg mixture, aiming to fill most of the cup while leaving a little room at the top for lift. Adding the remaining vegetables on top creates appealing color and gives a roasted edge to the veg as it meets the heat, while the crumbled feta cheese melts slightly and develops a golden speckled surface. You should see glossy egg liquid in each cup before baking. A pitfall here is overfilling the cups which can cause spillover and uneven baking, so stop short of the rim to allow for gentle expansion.
Bake the muffins for 20 to 25 minutes in the preheated oven until the eggs are set: As they bake, you will notice the edges turning a delicate golden, and the centers lose their glossy sheen, becoming matte and springy. The aroma will intensify, a warm, savory fragrance with hints of roasted vegetables and tangy feta cheese . To test doneness press the center lightly it should spring back without leaving an indent, and a knife inserted into the center should come out mostly clean. A typical mistake is removing them too early which leads to a wobbly center; if in doubt give them an extra 2 to 3 minutes. Overbaking will dry the eggs out, so watch for that subtle spring back.
Remove the baked egg muffins from the oven and let cool for a few minutes: The first few minutes out of the oven are critical because residual heat continues to set the custard, so allow them to rest on the counter. You will feel the heat radiating from the tin and you may notice the muffins contracting slightly as steam escapes, which is normal. This resting period also helps the muffins hold together when you remove them. A common error is attempting to extract them too soon, causing them to break apart; patience here makes a neater presentation.
Loosen the muffins with a knife, then remove them from the pan and let them cool slightly on a wire rack: Run a thin knife around each cup gently to release any edges that may have stuck, then lift them out and set on a wire rack so air can circulate underneath and prevent sogginess. The sound of a faint crackle and the sight of a slightly firm exterior signal readiness to handle. Cooling on the rack preserves texture and prevents steam from making the bottoms damp. If you skip the rack and set them on a plate they may sweat and lose the crisp edge; use the rack when possible.