Prepare air fryer: Preheat the air fryer to 400 degrees, according to the manufacturer’s instructions.
Whisk eggs, milk, salt, and pepper in a medium-to-large bowl.
Stir in spinach and mix until thoroughly combined.
Divide egg mixture evenly among ramekins, using a soup ladle.
Place ramekins in the air fryer basket or tray. Depending on the size of your air fryer, you may need to cook these in batches.
Cook the frittatas for 5 minutes. Remove the basket or tray from the air fryer and carefully top it with mozzarella cheese and bacon.
Return basket to air fryer and cook for 5-7 minutes longer, or until center of frittatas is 165 degrees F and cheese has turned golden brown.
Remove the air fryer basket or tray from the air fryer and remove ramekins carefully (rubber-tipped kitchen tongs work well here) to a heatproof surface.
Top with grape tomato halves and let stand for 3–5 minutes on a heatproof surface before serving