I really love this type of dish because it can be an appetizer or side dish depending on how you serve it. The nuts amplify the taste of the mushrooms and bring out the garlic and onions note in the mushrooms. These can be made in advance and simply heated and served when ready to eat.
For the mushrooms: Rough chop half of the mushrooms. Heat a large saute pan over medium heat. Add 2 tablespoons of the olive oil and when hot, add the diced onion. Season with salt and pepper and cook until tender, 8 to 10 minutes. Add the garlic and lower the heat. Add the chopped mushrooms and cook until the mushrooms are tender and most of the liquid has evaporated at the bottom of the pan, 8 to 10 minutes. Add half the white wine and cook until the flavor of the alcohol has mellowed considerably, 5 to 8 minutes. Taste for seasoning and adjust as needed. Add the sour cream, thyme and 1 tablespoon of the butter and allow it to melt over low heat. Remove from heat and reserve.
In a separate pan, heat the remaining 2 tablespoons olive oil over medium heat. When the oil is hot, add the whole stemmed mushrooms stem ends up. Saute for 5 to 8 minutes and then turn over and continue to cook for another 5 minutes. Add the remaining white wine and cook until the liquid has reduced by two-thirds. Add the remaining tablespoon butter and season with salt and pepper. Remove from the heat and reserve.
For the breadcrumbs: In a medium skillet over medium heat, add the butter and olive oil. Add the breadcrumbs and pecans and saute the mixture until it's golden brown, about 4 minutes. Season with salt. Set aside.
Bake and serve: Arrange the reserved stemmed and partially cooked mushrooms on a baking sheet and stuff them with the chopped mushroom and onion mixture. Bake for 12 to 15 minutes. Remove the mushrooms, set the oven to broil, top them with the breadcrumb mixture and place under the broiler until the tops are golden brown, about 1 minute. Serve immediately.