Wash meat, dab dry, cook in boiling salted water for about 12 minutes. Take the meat out and let it cool down. Cook pasta in boiling salted water according to package instructions until just firm to the bite. Brush a pan with oil. Leave the bacon in the pan for 3-4 minutes while turning it until crispy, then remove it
Peel and finely chop the onion. Melt 3 tablespoons of butter in a saucepan over medium heat. Fry the onion until transparent. Sprinkle with flour and sweat. Add milk bit by bit, stirring constantly. Season with pepper, salt and mustard. Simmer at low heat for about 5 minutes. Drain the pasta and let it drain well. Grate cheese, stir 3/4 into the béchamel sauce and let it melt. Wash the tomatoes, dab dry, cut in half. Clean, wash and chop the rocket. Dice chicken. Chop the bacon
Mix the pasta, chicken, tomatoes, bacon, rocket and sauce, pour into a greased casserole dish. Sprinkle with the rest of the cheese. Place 2 tablespoons of butter in flakes on top. In a preheated oven (electric cooker: 175 °C/ circulating air: 150 °C/ gas: see manufacturer)
Bake for 15-20 minutes