To make the meatballs, combine breadcrumbs, milk, and egg in a spacious bowl. Add beef, parsley, basil, and crumbled feta. Season with salt and pepper, mix thoroughly, and shape into 16 meatballs.
Preheat the air fryer to 200°C. Slide out the basket and line it with an air fryer parchment sheet or use baking paper to form a shallow base.
For the sauce, arrange garlic, cherry tomatoes, and shallots on the lined tray. Season with salt and pepper, then drizzle with olive oil and balsamic vinegar.
Air fry for 25 minutes, shaking the tray halfway through.
Once done, carefully transfer the contents to a large bowl, remove the paper, and blend the mixture until smooth using a stick blender or a food processor. Transfer the sauce to a small pan and keep it warm over low heat. Leave the air fryer on.
Place the meatballs evenly in the air fryer and cook for 10–12 minutes, ensuring they are cooked through.
Meanwhile, prepare the spaghetti according to the package instructions. Drain and mix in the butter.
For serving, heap the spaghetti on plates and place the meatballs and sauce on top. Garnish with chopped parsley, basil, and a sprinkle of crumbled feta.