Whisk the eggs. Crack the eggs into a small bowl. Add the heavy cream, salt, and pepper, and whisk vigorously for 30 seconds until pale and frothy.
Add the mix-ins. Stir in the diced bell pepper, red onion, and about 2/3 of the shredded cheddar. Reserve the rest of the cheese for the top.
Prep the ramekin. Grease a 7-ounce ramekin or small oven-safe dish thoroughly with cooking spray or butter — bottom and sides.
Pour and fill. Pour the egg mixture into the prepared ramekin. The mixture should reach no more than 2/3 of the way up.
Air fry. Place the ramekin in the air fryer basket. Cook at 350°F (175°C) for 10 to 12 minutes.
Add cheese on top. At the 8-minute mark, open the basket and sprinkle the reserved cheese on top. Close and continue cooking until the center is set and the top is lightly golden.
Rest and serve. Remove from the air fryer and let rest 1 to 2 minutes. Run a knife around the edge to release, then serve straight from the ramekin or invert onto a plate.