Not quick to make, but a meal in itself. The goal is a velvety, evenly coloured soup, so use a good heavy pot and take care during cooking to avoid it catching.
600 g thinly sliced onions (yellow or white), outer layer removed
3 tb butter
1 tb oil
1 tsp salt
3 tb flour
2 l stock
125 ml dry white wine
3 tb cognac
Heat butter and oil in pan until foaming has subsided (this means the optimum temperature is attained), add onions and cook slowly, covered, for 15 mins. Uncover, raise heat to moderate, add salt, cook 30 mins until onions are golden brown. Stir frequently. Add flour and cook 3 mins. Remove from heat and stir in stock and wine, season. Simmer partially covered for 30 mins. Add cognac right at the end.
And for the croutes:
4 slices french bread
Toast both sides of bread under the grill, rub one side with garlic, sprinkle with cheese, return to grill until melted. Place one on top of soup in each bowl, serve.
Source: Julia Child ‘Mastering the Art of French Cooking’