Prepare air fryer: Preheat air fryer to 400 degrees, according to manufacturer’s instructions.
Whisk eggs, milk, salt, and pepper together in a medium-to-large-sized bowl.
Stir in spinach and mix until thoroughly combined.
Divide egg mixture evenly among ramekins, using a soup ladle.
Place ramekins in 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 air fryer and carefully top with mozzarella cheese and bacon.
Return basket to air fryer and cook for an additional 5-7 minutes longer, or until center of frittatas are 165 degrees F and cheese has turned golden brown.
Remove air fryer basket or tray from 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 heatproof surface before serving.