Preheat and prepare: Heat your oven to 350°F (175°C). Grease a 12-cup muffin tin well with butter or oil. You can also use silicone liners for easy release.
Cook the bacon: Fry the bacon in a skillet over medium heat until crisp.
Drain on paper towels and chop into small pieces. Save a teaspoon of bacon fat for extra flavor if you like.
Whisk the eggs: In a large bowl, whisk eggs, heavy cream, garlic powder, onion powder, salt, and pepper until smooth and slightly frothy.
Add the mix-ins: Stir in chopped bacon, shredded cheese, spinach, bell pepper (if using), and green onions. Mix until evenly combined.
Fill the tin: Divide the mixture among the 12 muffin cups, filling each about 3/4 full.
Give the pan a gentle tap on the counter to settle the ingredients.
Bake: Place on the center rack and bake for 16–20 minutes, or until the muffins are set and the centers no longer look glossy. A toothpick should come out mostly clean.
Rest and release: Let the muffins cool in the pan for 5 minutes. Run a butter knife around the edges, then remove them to a cooling rack.
They’ll firm up as they cool.
Serve or store: Enjoy warm, or let them cool completely before storing.