Prepare the Ingredients: Clean and slice the mushrooms. Finely chop the onion and mince the garlic. Dice the bacon or speck if you're using it.
Cook the Bacon/Speck: In a large skillet over medium heat, cook the bacon or speck until crispy. Remove with a slotted spoon and set aside, leaving the fat in the pan.
Sauté the Vegetables: Add the butter to the hot skillet. Once melted, sauté the onion until translucent, then add garlic, 2 sprigs of thyme, and mushrooms. Cook until the mushrooms are golden and have released their moisture.
Make the Roux: Sprinkle the flour over the mushroom mixture, stirring well to combine. Add tomato paste and cook for a couple minutes, stirring continuously
Add Liquids: Gradually add the broth, and Worcestershire stirring continuously. Bring to a simmer and cook until slightly thickened.
Temper the Sour Cream: Before adding the sour cream, take a few tablespoons of the sauce from the skillet and mix it with the sour cream in a separate bowl. This gradual introduction of heat to the sour cream prevents it from curdling when added to the sauce.
Finish the Sauce: Stir the tempered sour cream into the skillet, mixing well to integrate. Avoid bringing the sauce to a boil at this point to prevent curdling. Season with salt and pepper to taste. Add the cooked bacon or speck back into the sauce. Simmer gently until sauce is to your desired thickness, add a splash of water, white wine, or broth if it becomes too thick.
Serve: Garnish with fresh chopped thyme and parsley before serving with your preferred meat or dish.