Prepare Oven & Roast Peppers : Position a rack in the center of your oven and preheat to 400°F (200°C). On the baking sheet, toss the diced bell peppers and shallot with 1 teaspoon of avocado oil, the smoked paprika, oregano, and black pepper. Spread into a single layer. Roast for 10–12 minutes, until the vegetables are tender and lightly browned at the edges.
Adjust Oven & Prep Pan : Once the vegetables are done roasting, reduce the oven temperature to 375°F (190°C). Lightly grease the 8-cup muffin tin with non-stick spray or a thin layer of oil.
Sauté Aromatics : While the peppers roast, warm the remaining 1 teaspoon of avocado oil in a 10-inch cast-iron skillet over medium-high heat. Add the chopped mushrooms and cook, stirring occasionally, for 4–5 minutes until they release their moisture and begin to brown. Add the packed spinach and minced garlic, stirring continuously for 1–2 minutes until the spinach has fully wilted. Remove the skillet from the heat.
Create the Egg Base : In a large mixing bowl, vigorously whisk the whole eggs and egg whites together for about 1 minute until the mixture is uniform, pale yellow, and slightly frothy.
Combine & Fill : Gently fold the roasted pepper mixture and the contents of the skillet (mushrooms, spinach, and garlic) into the egg base until just combined. Carefully pour the mixture evenly among the 8 prepared muffin cups, filling each about three-quarters full.
Bake to Perfection : Transfer the muffin tin to the oven and bake for 12–16 minutes. The egg muffins are ready when they are puffed and golden, and a toothpick inserted into the center of one comes out clean.
Cool & Garnish : Allow the egg muffins to cool in the tin for at least 5 minutes; they will deflate slightly as they cool. Run a thin spatula or knife around the edges to loosen, then remove them. Garnish with fresh chives, if desired, and serve warm.