Mushroom Soup

30min · 4 servings • 30g / 2 tbsp unsalted butter • 1 onion , chopped • 2 garlic cloves , minced • 400g / 14 oz white mushrooms • 200g/ 7 oz Swiss Brown/Cremini mushroom • 3 1/4 cups vegetable stock (bonus points for homemade veg stock! – Note 2), or chicken stock • 1/4 tsp salt , cooking/kosher • 1/8 tsp black pepper • 3/4 cup crème fraiche or cream