Combine ricotta cheese, Parmesan cheese, olive oil, lemon juice, lemon zest, grated garlic, salt, and black pepper in a large mixing bowl and stir until smooth.
Bring a large pot of salted water to a boil and cook spaghetti until al dente, about 9 to 10 minutes, reserving ½ cup of pasta water before draining.
Add fresh baby spinach to the boiling pasta water for 30 to 45 seconds to blanch until wilted, then drain pasta and spinach together.
Return the empty pot to the stove, add the ricotta mixture and ¼ cup reserved pasta water, stirring to create a smooth, thick sauce.
Add the drained pasta and spinach back into the pot and toss to coat evenly with the lemon ricotta sauce, adding more pasta water if needed to thin.
Transfer to a serving bowl and garnish with additional Parmesan, lemon zest, and cracked pepper before serving.