Slice the onion thinly. Peel the lower half of the asparagus spears and cut them into 2cm pieces. Dice the chicken. Cook the macaroni according to package instructions.

Melt butter in a frying pan. Sauté the onion until translucent, then add the chicken and cook until its color changes. Add flour and stir until the raw smell of flour disappears.

Add milk and stir well. Add the asparagus and cook until tender. Mix in the cooked macaroni, season with salt and pepper, and transfer to a gratin dish.