Cook pasta in boiling salted water until al dente; drain well and keep warm.
Heat oil in a large saucepan over medium heat. Add the mushrooms and cook for 5 minutes or until soft, stirring occasionally. Add onions, garlic, salt, pepper and thyme; cook 4 minutes, stirring occasionally.
Add the cream, increase the heat to high and bring to a boil, add the tomato paste, reduce the heat to medium low and simmer until thick, stirring constantly with a wooden spoon about 5 minutes. Add the parmigiano-reggiano and 1 tablespoon chives, stir well and adjust the seasoning to taste.
Add the pasta to the mushroom sauce, stirring well and cook until the pasta is heated through, about 2 minutes. Remove from heat. Sprinkle with remaining chives and serve immediately.