2 Chop 1/4 of the removed onion and use the rest for other purposes. Peel the potato and mash with a fork. Trim, rinse and cut the leeks into thin rings. Chop the cheese. Mix the chopped onion, potatoes, leeks and cheese together. Add 1 tablespoon of oil and season with salt and pepper.
Place the mixture into the onions and put them in a greased baking dish. Shower with nuts and drizzle a little oil over the onions. Bake on the middle shelf for about 30-40 minutes.
Serve with salad if desired.