To make the toppings, heat half the oil in a frying pan over a high heat, and cook the mushrooms for 10 mins until golden. Stir in half the garlic and the parsley, and fry for 1 min more. Season and scrape into a bowl. Heat the rest of the oil in the pan and fry the remaining garlic and the breadcrumbs for 3-5 mins, stirring until crisp. Scrape into a second bowl.
Melt the butter in a saucepan over a medium heat, then stir in the flour to make a thick paste. Gradually add the milk, a splash at a time, whisking until it’s all incorporated. Cook for 2-3 mins more until thickened. Meanwhile, cook the macaroni in a pan of boiling salted water for 5-6 mins. Stir the cheddar into the white sauce until all the cheese has melted, then fold in the macaroni.
Butter a 12-hole muffin tin and divide the mac ‘n’ cheese between the holes, packing it in with the back of a spoon. Top with the mushroom mixture, olives or sundried tomato slices, then scatter over the grated cheese and garlic breadcrumbs. Will keep, covered in the fridge, for up to 24 hrs.
Heat the oven to 220C/200C fan/gas 7. Bake for 15 mins until golden and piping hot. Leave to cool for 10-15 mins in the tin, then loosen by running a spoon around the edges. Transfer to a plate.