Prepare the chicken breasts by seasoning with black peppercorns, coarse salt, black pepper, and sweet paprika
Coat the seasoned chicken breasts with flour
Heat 50 g butter in a pan and cook the chicken breasts until golden
Sauté the onions and mushrooms in the same pan
Prepare the sauce by melting 50 g butter and whisking in 30 g flour to create a roux
Gradually add milk to the roux while stirring to create a smooth sauce
Season the sauce with salt and stir in the mustard
Add mozzarella and cheddar cheese to the sauce and stir until melted
Pour the cheese sauce over the cooked chicken, onions, and mushrooms
Serve hot
