Unless the pistachios are unsalted, put them in a sieve and rinse off the salt under cold water; dry them well on paper towels.

Heat 1 tbsp of the oil in a small skillet over medium heat. Add the pistachios and sauté for 2 minutes, stirring constantly. Drain on more paper towels.

In a food processor or blender, puree the nuts, the remaining 1/4 cup olive oil, the lemon juice and garlic. Add the cream and Parmesan and process for a few seconds more. Cover the mixture and let it sit. Meanwhile, cook the tortellini in plenty of boiling salted water according to the package directions, 7 to 9 minutes.

Drain the tortellini well. Return them to the pot and toss them with the pistachio sauce. Serve immediately, sprinkling each serving with chopped pistachios and grated cheese.