Fill a large pot with lightly salted water and bring to a rolling boil; stir in tortellini and return to a boil. Cook uncovered, stirring occasionally, until the tortellini float to the top and are tender yet firm to the bite, 2 to 3 minutes; do not overcook. Drain, reserving some pasta water.

Meanwhile, melt butter over medium heat in a large skillet. Add mushrooms, and sauté until tender and golden brown, about 5 minutes. Reduce heat to low, pour in 1 1/2 cups of cream, and simmer, stirring occasionally, 4 to 5 minutes.

Remove the skillet from the heat and add drained tortellini; toss to combine. Stir in remaining 1/2 cup of heavy cream, 1 1/2 cups Parmesan cheese, lemon zest, nutmeg, sea salt, and pepper; toss until well incorporated.