World's Greatest Onion Dip

INGREDIENTS

• 2 tablespoons olive oil
• 4 small onions, chopped
• 1/2 shallot, finely chopped
• 2 sprigs fresh thyme
• 2 tablespoons balsamic vinegar
• 2 tablespoons Worcestershire sauce
• 1/2 teaspoon coarse salt
• 1/4 teaspoon freshly ground pepper
• 1 (8-ounce) package cream cheese, softened
• 1/2 cup sour cream
• 2 tablespoons mayonnaise
• Pinch of celery salt
• Assorted crudite, for serving (optional)
• Flatbreads or toasted baguette slices, for serving (optional)