Heat the butter in a large pan and sauté together with the garlic and sliced onion for 20 minutes, until the onions have turned brown in colour and are soft and sweet. Pour over the red wine, beef stock and balsamic vinegar and season well with salt and pepper. Bring to the boil, then reduce the heat and simmer the soup for another 15 minutes. Meanwhile, you can prepare the Gruyere toast. Cut the baguette into 8 thick slices and toast them lightly on both sides under a grill. Pile the grated Gruyere` cheese on top of the toasted baguette slices and place them under the grill again till the cheese is bubbling. To serve, stir the chopped parsley into the cooked onion soup and serve in deep bowls. Place 2 pieces of the Gruyere` toast in each bowl and serve immediately.