Leek & Onion Soup

Ingredients

• 2 Tbsp unsalted butter
• 2 Tbsp olive oil
• 2 cups sliced leeks, white part only
• 4 cups assorted sliced onions—shallots, white and/or red onions
• Pinch of dried thyme or a sprig of fresh
• 1/2 tsp kosher salt
• 2 Tbsp cognac or dry sherry
• 6 cups beef or roasted chicken stock, ideally homemade
• 4 slices French bread, toasted if fresh
• 1 cup grated Gruyère cheese