Preheat oven to 400°F (200°C) and line a baking sheet with parchment paper.
In a large bowl, gently mix ground chicken, egg, panko, Parmesan, garlic, parsley, onion powder, paprika, salt, and pepper until just combined.
Shape the mixture into golf-ball-sized balls and arrange them on the prepared baking sheet.
Bake chicken bites for 20 to 25 minutes until cooked through and golden on top.
While chicken bakes, melt butter in a saucepan over medium heat; whisk in flour and cook for 1 minute.
Gradually whisk in milk until mixture is smooth and slightly thickened.
Stir in Parmesan, mozzarella, nutmeg, salt, and pepper until sauce is creamy.
Transfer baked chicken bites to a baking dish and pour the creamy sauce over them.
Broil for 3 to 5 minutes until sauce is bubbly and lightly golden.
Serve warm over mashed potatoes or cauliflower purée and garnish with fresh herbs.