In a skillet, melt butter over low heat and sauté thinly sliced onions over medium heat for 5 to 6 minutes, stirring frequently using a wooden spoon.
In the meantime, cut slices of French bread into 1/2 inch pieces, brush them with olive oil and toast them at 350 degrees in the oven for about 15 minutes until they are dry crusts.
Sprinkle flour over onion slices; stir until mixed well.
Slowly pour in brandy and dry white wine; bring to a boil, add water.
Lower heat to medium, cover skillet and cook for 45 minutes, stirring occasionally using a wooden spoon.
Remove from heat.
Season with salt and pepper.
Pour onion soup into individual bowls add bread, Swiss cheese and put under the broiler.