Cook tortellini according to package instructions.

Meanwhile, melt butter over medium heat; add flour and whisk until smooth. Add garlic and cook until fragrant, about 30 seconds. Slowly whisk in chicken broth, making sure no clumps form. Add milk and pumpkin and cook until thickened, about 5 minutes. Remove from heat and stir in Parmesan cheese and nutmeg.

Once tortellini is cooked, drain it and add to the sauce; toss to combine. Before serving, sprinkle with fresh sage and walnuts.