In a large skillet set over moderate heat 2 tablespoons of the butter and the oil until hot, add the shallot and cook, stirring, 3 minutes, or until softened. Add the mushrooms, thyme and salt and pepper to taste and cook the mixture, stirring occasionally, for 7 minutes, or until mushrooms are lightly golden. Add the prosciutto and peas and cook for 1 to 2 minutes, or until heated through. Add the cream and simmer just until lightly thickened.In a large pot of boiling salted water add the tagliatelle and cook it until al dente. Drain the pasta and transfer it to a serving bowl. Add the remaining butter to the pasta and toss to combine. Add the sauce and toss to coat the pasta. Serve with the Parmesan.
Recipe courtesy of Michele Urvater