Bring a large pot of salted water to a boil and cook pasta until al dente. Reserve 1/2 cup pasta water, then drain. In a large skillet over medium heat, heat olive oil and sauté minced garlic for 30 seconds until fragrant. Add fresh spinach and cook for 1–2 minutes until wilted. Reduce heat to low and stir in ricotta cheese, lemon juice, lemon zest, salt, and black pepper. Gradually add reserved pasta water, stirring until the sauce becomes smooth and creamy. Add drained pasta to the skillet and toss until evenly coated. Stir in grated Parmesan and red pepper flakes if using. Serve immediately.