In a large pot of boiling salted water, cook tortellini about 2 minutes less than package instructions. Add peas; cook until pasta is al dente and peas are tender, 2 minutes more. Reserving 1 cup pasta water, drain. Set aside pasta and peas.
Melt butter in pasta pot over medium-low heat. Add garlic; cook until fragrant, about 1 minute. Discard garlic.
To pasta pot, add pasta and peas, Parmesan, and 3/4 cup reserved pasta water; season with salt and pepper. Toss to coat. Add more pasta water, if necessary, to thin sauce. Serve, topped with additional Parmesan if desired.