Melt 4 tablespoons butter in a large Dutch oven or pot over medium-high heat. Add the onion, thyme, 1 teaspoon salt and a few grinds of pepper and cook, stirring occasionally, until the onion is very soft and a light golden brown, 10 to 15 minutes.  Add the bay leaf, garlic and wine to the pot and cook until the wine is almost completely evaporated, about 1 minute. Stir in the potatoes, chicken broth, the remaining 1 teaspoon salt and a few grinds of pepper and bring to a boil. Cook until the potatoes are very soft, 10 to 15 minutes. Discard the bay leaf. Use an immersion blender to puree the soup in the pot until smooth. (Or puree in a regular blender in batches, then return to the pot.) Stir in the heavy cream and remaining 4 tablespoons butter until melted. Preheat the broiler. Divide the soup among six 10-ounce ramekins. Place 2 slices of bread on top of each; sprinkle the bread and soup with the gruyère (about 1/2 cup per ramekin). Place on a foil-lined baking sheet and broil until the cheese is bubbly and starting to brown in spots, 2 to 3 minutes.