Melt the butter in a large skillet set over medium heat. Whisk in the flour until well combined. Slowly whisk in the milk. Add the garlic and cook, whisking constantly, until mixture comes to a simmer and lightly thickens. Stir in the walnuts and nutmeg, remove from the heat, cover and set aside.
Boil the tortellini until tender (check package for suggested cooking time). While the pasta cooks, uncover the walnut sauce, set back over medium heat and return to a simmer. When the tortellini is tender, drain well and add it, the spinach, cheese, salt and pepper to the skillet. Toss to combine and cook one or two minutes more, or until the spinach is just wilted. Divide the pasta among bowls and serve.
Note: The milk could be warmed in the microwave to just below the boiling point. To crush the walnuts, place them in a thick plastic bag and set on a cutting board. Use a kitchen hammer to pound and crush the walnuts.