Preheat the oven to 350°F.

Heat 1 Tbsp avocado oil in a large skillet over medium-high heat until shimmering.

Add 1 cup finely chopped yellow onion and sauté 2–3 minutes, until the onion begins to soften.

Add 1 cup finely chopped red bell pepper and 2 cups baby spinach; cook, stirring, until the spinach has wilted (about 1–2 minutes).

Stir in 6 ounces chopped cooked chicken breast and 1/2 cup sun-dried tomatoes; cook 30–60 seconds to heat through. Remove the skillet from the heat.

Spray a 12-cup muffin tin with cooking spray or grease the cups.

Divide the chicken and vegetable mixture evenly among the 12 muffin cups.

In a bowl or large measuring cup, whisk together 5 large eggs and 3/4 cup egg whites until combined. Add salt, pepper, or dried herbs if desired.

Pour the beaten eggs over the filling in each muffin cup, filling most of the way up but leaving a small gap at the top.

Sprinkle 1/2 cup feta cheese evenly over the tops of the cups.

Bake on the center oven rack for 20–25 minutes, until the egg cups are set and lightly golden (or until an instant-read thermometer inserted into the center reads 160°F).

Remove the muffin tin from the oven and let sit 5 minutes. Run a knife around the edges of each cup, then carefully lift the egg cups from the tin and let cool slightly before serving.