Heat the oil and butter in a deep saucepan and sautee the onions for a few minutes, stirring occasionally.
Reduce the heat and cook the onions for 20–30 minutes or until golden brown, stirring frequently to prevent burning.
Move the onions to a saucepan or casserole dish and pour over the stock. Season with pepper and simmer partially covered for 20 minutes.
Toast a few rounds of crusty bread for chunky croutons.
Spoon the soup into oven-proof serving dishes. Top each crouton with grated cheese and pop one or two into each bowl.
Place under the grill for a few minutes until golden brown. Serve immediately.
Recipe courtesy of Knorr