Preheat the oven to 400F degrees.
Clean and slice mushrooms. Line a baking sheet with parchment paper or foil, and arrange the sliced mushrooms on baking sheet. Drizzle ghee, and scatter the fresh thyme on top of the mushrooms. Toss to combine. Sprinkle with sea salt and pepper.
Roast for 20 minutes. Remove from oven, give the mushrooms a stir, and roast another 10 minutes. Remove from oven, set aside.
Spread ghee or unsalted butter onto each slice of sourdough, and toast for 1-2 minutes in oven.
Remove the toast from the oven top with Fontina. Bake 2-3 minutes, until cheese has melted.
Remove the baking sheet from the oven and top each piece of bread with a spoonful or two of the mushroom mixture. Garnish with fresh thyme, if desired and enjoy!