Put the sprouts in a pot full of water. Boil them until tender. Drain and transfer them immediately into a large bowl with very cold water.

When the sprouts are boiling, prepare the creamy sauce. Melt butter in a pot. Add in flour and saute it on the lowest heat stirring continually until it changes the color. Pour the milk into the pot and stir fast. Cook until thickens. Add grated onion and ¾ of the grated parmesan into the sauce, stir well until the cheese melts and put it aside.

Brush a casserole dish with olive oil. Place diced potatoes at the bottom. Toss in the brussles sprouts. Sprinkle salt over them. Pour the creamy cheesy white sauce over them. Bake in preheated oven at 390F (200C) for 40 minutes. Take it from the oven and sprinkle the rest of the parmesan and bak efor another 5 minutes until the cheese on the top melts.

Sprinkle black pepper right after you remove it from oven.