Finest French Onion Soup

Ingredients (for 4 servings)

• 5 whole — Red onions
• 4 tbsp — Cultured butter
• 2 l — Chicken or beef stock
• 1 tbsp — Fish sauce
• 2 tsp — Apple cider vinegar
• 1 tsp — Thyme
• 0.5 cup — Gruyere cheese