Bring several cups of salted water to a boil & add the agnolotti. Cook for about 7 minutes, drain, add a couple glugs of olive oil to prevent from sticking, and set aside. In a medium saucepan, heat the cream and butter together until the butter is completely melted. Add the asiago cheese in small handfuls, whisking well to make sure the cheese melts evenly. When all the cheese has been added, heat for another 2 minutes over medium heat until completely heated through (sauce might bubble a bit; that's okay, just don't let it get to a full boil to prevent from sticking/burning). Add plenty of fresh ground pepper and stir well. Finally, add the mushrooms and remove from heat. The mushrooms will be cooked by the heat of the sauce.
In a bowl or separate saucepan, combine the agnolotti and the sauce with mushrooms. Add the fresh spinach and stir gently to combine. The spinach will wilt slightly in the sauce but not be completely cooked (i.e. mushy). Sprinkle extra grated parmesan or asiago cheese, serve with a salad, and enjoy!