Creamy White Onion Soup

Name: Creamy White Onion Soup
Ingredients: 2 shallots, thinly sliced into rounds; ¼ cup apple cider vinegar; ¼ cup white wine vinegar; ¼ cup sugar; 1 Tbsp. olive oil; 1 medium red onion, thinly sliced; Kosher salt; ½ cup red wine; ¼ cup red wine vinegar; 1 Tbsp. sugar; ¼ cinnamon stick; ½ star anise pod; ⅛ tsp. crushed red pepper flakes; ⅛ tsp. ground allspice; ¼ cup olive oil; 4 large white onions, thinly sliced; 1 leek, white and pale-green parts only, thinly sliced; 2 celery stalks, chopped; 4 garlic cloves; 1 serrano chile, seeded, chopped; 2 Tbsp. chopped peeled ginger; ½ cup dry white wine; ½ cup mirin; 1 cup heavy cream; 2 Tbsp. unsalted butter; 2 slices brioche, quartered; 4 oz. Fontina cheese, grated; 1 Tbsp. chopped fresh chives