World's greatest onion dip

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)