In a large pot of salted boiling water, cook spaghetti according to package instructions until al dente. Drain, reserving 1 cup pasta water, and return to pot. Meanwhile, in a large skillet over medium heat, melt butter. Add garlic and stir until fragrant, then add asparagus and cook until just tender, 4 to 5 minutes. Add shrimp and cook until pink, 3 minutes more. Season with salt and pepper. Add lemon juice, lemon slices, Parmesan and heavy cream and let simmer, then add cooked spaghetti. Toss until coated, then add pasta water as needed until sauce is creamy.