Bring a large pot of salted water to a boil, cook the tortellini until done, approximately 8 minutes. Drain and set aside.
While the pasta is boiling, brown the sausage in a large skillet over med-high heat. Transfer the cooked sausage to a plate using a slotted spoon and drain off all but two tablespoons of fat. Add the onion to the pan and sauté until soft and golden. Add the garlic and cook for one minute. Add the sausage back to the pan and stir to combine. Add the pasta and pesto and toss gently to combine.
Top with grated parmesan cheese and toasted pine nuts. Serve immediately