Preheat oven to 375 degrees with the rack in middle.
Arrange slices on a large baking sheet and bake in oven until golden, about 8 minutes. Let cool.
Meanwhile, heat the butter and olive oil in a heavy skillet over medium heat until butter is melted, add onion, garlic clove, bay leaf, and thyme, and cook covered, stirring occasionally, for 10 minutes.
Uncover skillet and cook, stirring occasionally, until deep golden, about 10 to 15 minutes more. Side Note: If the onions get dry and start to burn, add a few tablespoons of beef broth or water.
Add wine and cook until entirely evaporated, 1 to 2 minutes.
Discard garlic clove, bay leaf, and thyme.
Remove from the heat then season with sea salt and freshly cracked pepper, to taste.
Preheat the broiler and move a rack 3 to 4 inches from the top.
Arrange crostini on a large baking sheet, then top each with some of the caramelized onions and sprinkle generously with shredded Gruyere.
Place into the oven and broil until cheese is melted and gooey, 1 to 2 minutes (watch carefully). Top with a bit of fresh thyme. Enjoy!