Cook the tagliatelle al dente. Meanwhile, heat the oil and butter in the pan. Add finely chopped onion and sauté. Add the mushrooms cut into strips. Pour the lemon juice over them. Cook for 10 minutes over medium heat with the pan uncovered. Just before the end, add the finely chopped garlic cloves. Remove from heat, add salt and pepper to taste and stir in the finely chopped parsley.
Drain the tagliatelle and mix with the sauce. Serve immediately. As a wine, I recommend Lagrein dark from South Tyrol.