In a pot, heat the olive oil over medium heat, and cook and stir the onion until slightly browned, about 10 minutes. Stir in the mincemeat, and cook and stir until mincemeat is hot and well mixed with onion. Pour in the pasta sauce and tomato puree, and mix in the basil leaves. Bring the sauce to a simmer, and cook until the flavors have blended, about 10 minutes. Stir often.