1. Melt 1 tablespoon butter in a medium pot over medium-high heat. Stir in chopped onion, season with salt, cover the pot and sauté the onion for a couple of minutes. Then add a pinch of sugar, reduce heat to real low and cook for about 25-30 minutes, until well browned but not burnt (stir every few minutes).
2. Add 1/2 tablespoon butter to the pot, then add flour and stir well.
3. Gradually whisk in the wine and beef broth, stirring constantly. Add bay leaf, season with salt and pepper to taste, and cook for 10 more minutes.
4. Add Sherry vinegar and more salt, if necessary.
5. Preheat oven to 450F (230C).
6. Top the bread slices with some butter, sprinkle with salt and pepper. Bake in the preheated oven for 1-2 minutes, then remove from the oven, top with cheese and bake for another 2 minutes.
7. Preheat broiler. Divide the soup between 2 french onion soup bowls (or ramekins). Top generously with cheese and broil for 1-2 minutes to brown the top.