Preheat the oven to 400 degrees F. Heat the oil over medium heat in a large skillet. Add the onions and thyme and cook, stirring occasionally, until onions are very soft and browned, about 15 minutes.
Meanwhile, stir together the sour cream, mayonnaise and Worcestershire sauce; season with salt and pepper. Fold in the onion mixture until well combined. Cover and chill for at least 1 hour or up to overnight. Mixture can be stored in an airtight container for up to 3 days.
Arrange the bread in an even layer on a baking sheet. Sprinkle each slice with about 1 tablespoon cheese. Bake until the bread is golden brown and crisp and the cheese is melted, 6 to 8 minutes. Serve dip with the cheese toasts and endive leaves.
Recipe courtesy Food Network Kitchens