Combine the crumbled sausage, egg, oats, cayenne pepper, salt, and black pepper in a mixing bowl.

Mix well and shape into meatballs.

Preheat the air fryer to 380°F (193°C).

Spritz the basket with cooking oil and cook the meatballs for 10 minutes, shaking the basket halfway through cooking.

Reserve the meatballs.

In a pan over medium-high heat, heat olive oil and sauté the chopped leek until tender and aromatic.

Stir in the garlic, chili pepper, oregano, basil, and celery seeds.

Cook for 2 minutes.

Add the brown mustard and tomato puree to the pan, stirring well.

Let the sauce cook for 5 minutes.

Transfer the tomato sauce to a baking pan and nestle the reserved meatballs into the sauce.

Place the baking pan in the air fryer and cook at 350°F (175°C) for 10 minutes.

Serve the meatball casserole warm.

Enjoy!