Make the soup: Peel the onions, cut in half and into thin slices. Melt the butter in a large pan and fry the onions until brown. Add the garlic and cook for about 20 minutes. Add the flour and the tomato paste, stir continuously. Remove from heat, pour the cold stock, add salt and pepper, and stir until the flour is fully mixed in. Return to heat and bring to a boil. Add the bay leaves, thyme and basil. Reduce heat and simmer, uncovered, on a low heat for 30 minutes.

Make the garnish and serve: While the soup is simmering cut the baguette into slices. Grill the bread until it is toasted. Put the soup in serving bowls and put one or two slices of bread on top; cover with grated cheese. The bread should be totally covered. Place the bowls under the grill of the oven until the cheese is melted and slightly brown. Serve immediately.