Chop the onion. Heat the oil in a pan over medium heat and fry the onion for about 5 minutes until browned. Add the flour and stir for 1 minute to form a paste. Gradually add the cold milk while stirring constantly with a whisk until the mixture reaches medium consistency and comes to a boil.
Remove the pan from the heat. Stir in the Cremefine. Chop the 2 garlic cloves and add them to the sauce. Add the broth cube and season with salt, pepper, and Provence herbs to taste.
Season the chicken breast fillets with salt, pepper, and paprika powder. Place them in a baking dish and pour the sauce over them.
Bake at 180°C (350°F) for 25 minutes. Remove from the oven and top with the Gouda and Emmentaler cheese slices. Bake for another 35 minutes until the cheese is melted and lightly browned. Check that the thickest part of the chicken has reached an internal temperature of 74°C (165°F).