Preheat the oven. Set it to 400°F (200°C). Lightly grease a 12-cup muffin tin with cooking spray or butter to prevent sticking.
Par-cook the bacon. Lay bacon strips on a sheet pan lined with parchment.
Bake for 8–10 minutes until some fat renders and the bacon is pliable but not crispy. This helps it hold shape in the muffin cups.
Line the muffin cups. Once cool enough to handle, wrap a bacon slice around the inside edge of each muffin cup to form a ring. If slices are short, overlap two pieces.
You can also place a small piece at the bottom for extra coverage.
Mix the egg base. In a bowl, whisk eggs, milk, salt, pepper, and garlic powder until smooth and slightly frothy. Stir in cheddar, Parmesan, green onions, and bell peppers if using.
Fill the cups. Divide the egg mixture evenly among the bacon-lined cups, filling each about 3/4 full. The eggs will puff a bit as they bake.
Bake. Reduce oven temperature to 375°F (190°C).
Bake for 12–15 minutes, or until the centers are just set and a toothpick comes out mostly clean. The tops should look slightly puffed and glossy, not dry.
Rest and release. Let the cups sit for 3–5 minutes to firm up. Run a thin knife around the edges and gently lift each cup out.
The bacon should be crisp around the edges.
Garnish and serve. Top with extra green onions, a sprinkle of Parmesan, or hot sauce. Serve warm.