Cook turkey sausage for 4 minutes in a pan over medium heat, using a wooden spoon to break the sausage into smaller pieces.
Add diced onion and pepper and continue cooking for another 4 minutes, stirring frequently.
Remove from heat and let cool.
Add eggs and milk to a bowl and whisk vigorously to combine.
Spray each ramekin with cooking spray. Divide egg and milk mixture evenly between 6 ramekins.
Divide turkey sausage mixture evenly between 6 ramekins (around 2 large spoonfuls per ramekin).
Add a generous pinch of shredded cheese to the top of each ramekin.
Air fry at 325 degrees Fahrenheit for 20 minutes. Remove the egg muffins from the air fryer and let cool for 2-3 minutes before serving.
Serve with salsa or hot sauce.