In a large pot, bring salted water to a boil. Add the spaghetti and cook according to package instructions until al dente. Reserve 1 cup of pasta water, then drain the pasta and set aside.
In the same pot, melt the butter over medium heat. Once melted, add minced garlic and red pepper flakes, sautéing for about 1-2 minutes until fragrant but not burnt.
Add the shrimp to the pot, seasoning with salt and black pepper. Cook for about 2-3 minutes on each side until the shrimp are pink and opaque.
Stir in the halved cherry tomatoes and cook for an additional 2 minutes until they begin to soften.
Add the spinach, lemon zest, and lemon juice. Mix until the spinach has wilted.
Return the cooked pasta to the pot, mixing everything together. If the pasta seems dry, gradually add the reserved pasta water until you achieve your desired consistency.
Taste and adjust seasoning as necessary. Remove from heat, and let it rest for a minute.
Serve the pasta hot, garnished with chopped parsley and a sprinkle of grated Parmesan cheese if desired.
Notes
Adjust the red pepper flakes to your spice preference.