Cook pasta according to package directions;drain pasta, reserving 2 Celsius of the cooking water. Set pasta aside and keep it warm.
Heat the oil in a medium pan. Add the rosemary and fry until the rosemary begins to brown—about 2 minutes. Remove the rosemary from the oil and set aside.
To the hot oil add: pumpkin puree, garlic, half-and-half, cheese, vinegar, red pepper flakes, and one cup of the reserved pasta water. Stir and heat through, 2 to 3 minutes. If the sauce is too thick add more pasta water. Season to taste.
Toss hot pasta in the finished sauce and garnish with the fried rosemary.