Melt the butter in a saucepan over medium heat. Stir in the flour and cook for 1–2 minutes.
Gradually whisk in the milk, then add the salt and black pepper. Cook for 2–3 minutes until thickened.
Stir in the cream, cook for 1 minute more, and set aside.
Heat the oil in a skillet and cook the onion until translucent.
Add the chicken, bell pepper, red chili flakes, black pepper, and salt. Cook for 2–3 minutes.
Stir in the cream and cook for 2–3 minutes. Set aside.
Fill each tortilla with the chicken filling and roll it up tightly.
Spread some sauce in a baking dish, place the enchiladas seam-side down, and cover them with the remaining sauce. Top with mozzarella.
Bake at 320°F (160°C) for 5–6 minutes in a pre-heat oven until the cheese melts. Sprinkle with red chili flakes and serve.