This might be one of the creamiest light pasta dishes you've ever had! The mascarpone cheese adds a richness to the spinach pasta that's unexpected--especially in a healthy pasta recipe.
Cook pasta according to package directions, omitting salt. Drain, reserving 1 cup cooking water.
Transfer pasta to a large bowl; add garlic, shallot, mascarpone, spinach, salt, pepper, and 1/2 cup of the reserved cooking liquid. Stir until cheese has melted and mixture is combined, about 1 1/2 minutes. Add additional cooking liquid as needed to loosen sauce. Divide pasta among 4 bowls. Sprinkle with lemon zest and, if desired, crushed red pepper. Serve immediately.