In a large sauté pan over medium-high heat, add the olive oil and then the pancetta. Cook the pancetta until crisp and the fat has rendered, then add the mushrooms. Toss the mushrooms and then sauté until browned and softened. Add the garlic, cook for 30 seconds, then add the pasta, with a bit of pasta cooking water to loosen the sauce. Add the butter, tarragon, and toss everything to coat and melt the butter and then transfer to a platter and serve. Garnish with parmigiano reggiano.