Spray an 8-inch silicone tray with cooking spray.
In a mixing bowl or glass measuring cup, whisk together eggs and a splash of milk (or water). Whisk vigorously with a fork until frothy. This will introduce air into the mixture, making your omelette fluffy.
Add diced ham, bell pepper, green onions, chopped spinach, and grated cheese to the beaten eggs. Season with salt and pepper, and mix well.
Transfer the egg mixture to the prepared silicone tray. Even out with a fork if needed. Top with sliced cherry tomatoes.
Preheat the air fryer to 340°F for a few minutes. Take the basket out completely, carefully place the silicone tray inside, place the basket back in the air fryer, and air fry for 9-12 minutes.
Check the omelette after 9 minutes. Lift the edge gently to ensure it's set. If it's still undercooked or if you see any liquid, air fry for 2-3 minutes longer until fully set.
Remove the silicone tray from the air fryer and place it on a plate or heat-proof surface. Gently lift the omelette with a spatula and slide it onto a plate, being careful not to break it.
Serve hot on its own or with your favorite breakfast sides.