In a large skillet, heat 2 tablespoons of the oil. Add mixed mushrooms and cook over medium high heat until mushrooms begin to brown.
Add the onion and season with salt and pepper. Lower the heat. Cover and cook for about 5 minutes. Stir in the garlic, cover and cook for another 2 minutes.
Add stock, whisk in mustard and heavy cream.
Simmer over low heat, stirring, until thickened, about 4 minutes. Adjust salt and pepper to taste.
Transfer cooked buckwheat to a serving dish.
Add mushrooms and fresh parsley. Combine gently and serve.