Heat oven to 400°F. Line a rimmed half sheet pan with parchment and brush with olive oil.
Whisk eggs, milk, salt, pepper, garlic powder, and paprika in a large bowl until smooth and frothy.
Spread peppers, onion, mushrooms, and spinach on the prepared pan in an even layer.
Scatter the chopped bacon over the vegetables.
Pour the egg mixture evenly over the pan. Tilt gently to coat the surface.
Sprinkle cheddar and parsley on top. Tap the pan once to release air bubbles.
Bake 14–18 minutes, until set in the center and lightly golden at the edges.
Rest 2–3 minutes. Slice into squares, garnish with extra parsley, and serve warm.