Brush dirt from mushrooms and slice. In medium skillet over medium heat, warm butter with olive oil. Add garlic and guanciale and sauté just until meat turns pink, about one minute. Add sliced mushrooms and sauté for about two minutes, stirring constantly. Be sure that the butter and oil do not get too hot and fry, rather than cook, the mushrooms. When mushrooms are almost cooked through, slowly add cream and stir until sauce reaches desiredconsistency. Season with salt and pepper. Remove from heat and cover to keep warm.
In a medium pot over high heat, cook tagliatelle in boiling water until al dente, about two minutes for fresh pasta and seven to eight minutes for dry. Drain pasta and toss with sauce. Garnish with parsley and fresh ground pepper. Serve warm.