Preheat the oven to 375°F (190°C).
Lightly grease a 12-cup muffin tin with nonstick spray or a thin layer of oil.
Par-cook the bacon. Lay bacon on a sheet pan lined with parchment. Bake for 8–10 minutes until just starting to render and turn golden but still pliable. Do not fully crisp.
Line the cups. Let bacon cool slightly, then wrap one slice around the inside wall of each muffin cup to form a ring.
If using thinner bacon, overlap slightly or add a half strip to reinforce.
Make the egg mixture. In a bowl, whisk eggs, milk, salt, pepper, and smoked paprika. Whisk just until combined—no need to whip lots of air in.
Add mix-ins. Stir in cheese, bell pepper, onion, and spinach. Distribute the mix-ins evenly so each bite gets a little of everything.
Fill the cups. Pour the egg mixture into each bacon-lined muffin cup, filling about 3/4 full.
If you have extra, top off the sparsest cups.
Bake. Place the muffin tin on the middle rack and bake 12–16 minutes. They’re done when the centers are set and no longer jiggly. A toothpick should come out mostly clean.
Rest and release. Cool for 3–5 minutes.
Run a thin knife around the edges to release, then lift out gently.
Finish and serve. Sprinkle a pinch of extra cheese or chopped herbs on top if you like. Serve warm.