Whisk the olive oil and flour together in a saucepan until smooth, then warm over a medium heat until lightly golden and thickened slightly. Add the milk a splash at a time, whisking continually, until a smooth white sauce forms. Cook for 2 mins more until thick enough to coat the back of a spoon. Stir in 100g of the Parmigiano-Reggiano and the Dijon mustard, then cook for another 1-2 mins until melted. Season well.
Meanwhile, cook the pasta for 8-10 mins until al dente. Drain well and stir into the cheese sauce. Tip into a baking dish and heat the grill to medium. Toss together the breadcrumbs, paprika or cayenne and the remaining Parmigiano-Reggiano, then scatter over the top of the macaroni cheese. Grill for 5-8 mins until the top is crispy and golden. Serve with a sharply dressed green salad.
