Preheat the oven to 200°C (400°F).

In a large bowl, combine the ground chicken, egg, breadcrumbs, grated Parmesan, minced garlic, chopped parsley, onion powder, paprika, salt, and pepper. Mix gently until just combined.

Shape the mixture into medium-sized balls and arrange them on a parchment-lined baking tray. Bake for 20-25 minutes until golden brown and cooked through.

While the meatballs bake, melt butter in a saucepan over medium heat. Whisk in the flour and cook for 1 minute to form a roux. Gradually add milk, whisking constantly until smooth and slightly thickened. Stir in grated Parmesan, shredded mozzarella, and season with salt, pepper, and nutmeg.

Transfer the baked chicken bombs into a small casserole dish and pour the prepared Parmesan sauce over them. Broil for 3-5 minutes until the sauce is bubbling and lightly golden.

Spoon mashed potatoes or cauliflower purée onto plates or bowls. Arrange the chicken bombs on top and drizzle with extra sauce from the casserole dish. Garnish with fresh thyme or parsley and optionally drizzle with olive oil or butter.