This is fairly time consuming as the onions do have to be cooked till they are gorgeously caramelly and sticky. But once that's done, the rest of the soup is remarkably easy to put together and uses minimal ingredients. I tend to use the healthier olive oil instead of butter, but if you prefer, you can use butter to caramelise the onions.
This recipe is from Raymond Blanc, but I have adapted it to suit us. I still use the wine (even with a little one, as the alcohol is boiled away), but you can substitute a medium sherry instead which adds more sweetness to the finished soup. We eat this with French bread and grilled cheese and its a lovely, satisfying lunch!!
4 tbsp olive oil
1 tbsp butter
Around 1/2 kilo of Spanish onions, sliced
2 tbsp plain flour
100 ml dry white wine (boil to get rid of the alcohol)
Around 900 ml vegetable stock
1 tsp brown sugar
Salt (adjust, as the stock can be salty)
Slices of French Bread
Put the olive oil and butter into a heavy pot, heat for a few minutes and toss in the onions. Cook, stirring ocasionally for about 30 - 45 minutes, don't rush this part, as the onions do need to be caramel and sticky and sweet. The colour is up to you, I like it fairly light, but if you want a dark colour keep cooking. Take care not to burn the onions. Add the sugar and flour and cook for another 5 minutes.
Add the white wine and the stock, and bubble for another 10 minutes or so. Season with the salt and pepper. Grill the cheese on the French bread. Ladle the soup into bowls and top with the toasted bread and cheese.