In a large pan, heat the butter until bubbling over a medium/high heat.Add the onion, thyme and bay leaf and stir well. Reduce the heat to medium/low and let them gently fry for 30 minutes, stirring regularly. They will soften and turn golden brown.
Add the garlic and fry for 1-2 minutes before adding the stock and soy sauce (if using). Season with the salt & pepper. Bring the soup to a boil then reuce the heat to low and simmer gently, uncovered for 30-40 minutes. Just bubbling.
Fish out the bay leaf and any thyme stalks and remove from the heat.
Preheat a grill/broiler until hot.In a toaster, toast the bread lightly. Meanwhile, spoon a portion of soup into a small bowl and place a slice of toast on top. Pile on about 1/2 cup cheese on the bread then place under the hot grill/broiler for 3-4 minutes until the cheese is completely melted and bubbling.Repeat with more soup/bread/cheese combos.Serve hot!