Preheat your air fryer: Set it to 350°F 175°C for 3 to 5 minutes. A hot air fryer helps the eggs set and the top brown evenly.
Prep the pan: Lightly grease your casserole dish or loaf pan with cooking spray or a thin layer of olive oil to prevent sticking.
Cook the meat if raw: Brown sausage or bacon in a skillet until fully cooked and slightly crisp. Drain extra fat. If using ham, just dice it.
Par cook potatoes if needed: If using frozen hash browns, no need to thaw. If using fresh potatoes, cook them briefly in the microwave or skillet until just tender.
Whisk the egg mixture: In a large bowl, whisk eggs, milk, garlic powder, smoked paprika, salt, and pepper until smooth and slightly frothy.
Build the casserole: Add the meat, potatoes, bell pepper, onion, and spinach to the greased dish. Sprinkle half the cheese. Pour the egg mixture over the top and finish with the remaining cheese.
Air fry: Place the dish in the air fryer basket. Cook at 325 to 330°F 165 to 170°C for 22 to 28 minutes, depending on your air fryer and dish depth. The center should be just set and no longer jiggly.
Check doneness: Insert a knife or toothpick into the center. It should come out mostly clean. If the top is browning too fast but the center is not set, cover loosely with foil and continue cooking 3 to 5 more minutes.
Rest and serve: Let the casserole rest 5 minutes. This helps it firm up and slice cleanly. Cut into squares and serve warm.
