In a food processor, combine spinach, basil, walnuts, 3 tablespoons olive oil, lemon juice and zest and ¼ cup water, then puree until smooth. Season with salt and pepper, then set aside.
In a high-sided skillet or wide braiser, heat remaining olive oil, garlic and crushed red pepper over medium-high heat. Cook, stirring constantly, until golden and fragrant, about 2 minutes. Add the pasta, 5 cups water and a large pinch of salt. Bring to a simmer and cook until the pasta is al dente and the water has reduced into a thick sauce, 13-15 minutes.
Remove from the heat and stir in the spinach pesto. Adjust seasoning with salt and pepper, then serve.