Step 1

Stir and smash 1 stick room-temperature unsalted butter, 2 tablespoons mayonnaise, 1 tablespoon plus 1 1/2 teaspoons onion soup mix, and 2 minced garlic cloves together with a flexible spatula in a small bowl until combined.

Step 2

Heat the oven to 375ºF. Line a baking sheet with parchment paper or aluminum foil.

Step 3

Use a serrated knife to split 1 loaf French bread in half. Place cut side up on the baking sheet. Spread the butter mixture evenly onto the bread.

Step 4

Sprinkle evenly with 3 ounces shredded mozzarella cheese, 3 ounces shredded Gruyère cheese, 2 teaspoons fresh thyme leaves, and 1/4 teaspoon black pepper.

Step 5

Bake, rotating halfway through, until the cheese is melted and the top is browned in spots, about 15 minutes total. Let cool for at least 10 minutes. Garnish with more fresh thyme leaves if desired and cut crosswise into pieces.