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.