Preheat oven to 375°F (190°C). Dice the onion and bell pepper, slice the mushrooms, and roughly chop the spinach if leaves are large. Shred the cheese.

Heat olive oil in a 10-inch oven-safe skillet over medium heat. Add diced onion and bell pepper, cook for 3-5 minutes until softened. Add sliced mushrooms and continue to cook for another 3-4 minutes until mushrooms release their liquid and brown slightly. Stir in the baby spinach and cook until wilted, about 1-2 minutes. Remove skillet from heat and evenly spread the vegetables across the bottom.

In a large mixing bowl, whisk together the eggs, milk (or half-and-half), salt, and black pepper until well combined and slightly frothy.

Pour the egg mixture evenly over the cooked vegetables in the skillet. Gently sprinkle the shredded cheddar cheese over the top of the egg mixture. Place the skillet back on the stovetop over medium-low heat for 2-3 minutes, just until the edges of the frittata begin to set.

Transfer the skillet to the preheated oven. Bake for 18-20 minutes, or until the frittata is set in the center and lightly golden brown on top. A knife inserted into the center should come out clean.

Carefully remove the frittata from the oven. Let it rest for 5 minutes before slicing into wedges and serving warm.