Mushroom Parmentier

Name: Mushroom Parmentier
Ingredients: 2 lbs mushrooms, any combination you like; 3 tablespoons butter; 2 tablespoons olive oil, more as needed; 1 large red onion, chopped; 3 garlic cloves; 3 -4 sprigs fresh thyme leaves, stems removed; 1/2 cup red wine; 1/2 cup vegetable stock or 1/2 cup water; salt and pepper; 1 tablespoon flour; 1/4 cup fresh parsley, chopped; 6 medium floury potatoes, such as Yukon gold; 2 tablespoons butter; 1/4 cup milk; 4 ounces gruyere cheese, cut into very fine dice; 2 tablespoons parmesan cheese