Using a broiler or grill, toast the baguette slices until golden brown, or desired doneness.
Heat a skillet over medium heat. Add the butter and olive oil. When hot, add the shallots and onion. When translucent and beginning to brown, add the mushrooms.
Once the mushrooms have wilted, about 3-4 minutes, add the salt and pepper to taste. Add the sour cream and stir to combine. Take off the heat and spread a generous amount onto each piece of toast.
Garnish with parsley if desired. Serve immediately.