1) Toast hazelnuts in a dry skillet over medium heat until fragrant, about 3 minutes.
2) Cook pasta in salted water until done (about 1 tablespoon kosher salt per gallon).
3) Meanwhile wash spinach well.
4) Stir together mascarpone, lemon juice, salt, and nutmeg.
5) When pasta is done, reserve about 1 1/2 cups of the pasta water.
6) Drain pasta and then add pasta back to pan. Pour over mascarpone sauce as well as about 1/2 cup of pasta water. Stir pasta over low heat.
7) Stir in spinach and continue stirring until spinach is wilted, about 2 minutes.
8) Sauce should thicken as it cooks, if it is too thick than add more pasta water by the tablespoon.
9) Serve pasta immediately garnished with toasted nuts.