Preheat oven to 400 F and line a large baking sheet with parchment paper (must be parchment)

Scatter mushrooms, leeks and garlic on the prepared baking sheet

Roast for 20 minutes. Using a spatula loosen the mushrooms and leeks off the parchment paper and set aside

Bring 8 cups of water to boil. Add thyme and mushroom mixture, reduce heat to low and cover. Simmer for 40 minutes. Strain broth