Melt the butter with a little oil in a large frying pan and fry the onion and garlic until translucent.*It's a good idea to use a pan that can go in the oven, as the dish goes into the oven for 15 minutes before serving.
Add the taco seasoning, jalapeños and tomato purée and heat through for a couple of minutes.
Sprinkle the flour over and stir it in well, then add the milk while stirring thoroughly.
Let the sauce simmer for 5-10 minutes. Remember to stir the sauce regularly so it doesn't catch on the bottom. Season to taste with salt and freshly ground pepper.Meanwhile, cook the pasta until al dente.
When the pasta is almost done, stir the cheese into the sauce. Keep back a good handful – about 100 g – for sprinkling on top.
Add the peppers and beans to the sauce and stir well.
Drain the pasta and fold it into the sauce. Adjust the consistency with a splash of the pasta water if it seems too thick.*If your pan is ovenproof, leave the dish in it – otherwise transfer it to an ovenproof dish.
Scatter the remaining cheese over the top and bake in a preheated conventional oven (top and bottom heat) at 230°C for about 10-15 minutes, until golden and bubbling.
Let the dish rest for 10 minutes before serving, then top with finely chopped parsley, fresh jalapeños or similar.Serve with ice-cold crème fraîche.