Bring a large pot of salted water to a rolling boil. Add the fresh spinach and blanch for 30 seconds until it wilts and brightens. Transfer to an ice bath, cool, drain, and squeeze excess water.

In a large skillet, heat 2 tablespoons of olive oil over medium heat. Sauté finely chopped yellow onion for 10-12 minutes until soft. Add minced garlic, cooking for another minute until fragrant.

Bring fresh salted water to a boil in the same pot. Add tagliatelle, cooking according to package instructions (8-10 minutes). Reserve 1 cup of starchy pasta water before draining.

In a blender, combine blanched spinach, basil, parsley, pistachios, and lemon juice. Blend until smooth, then gradually add olive oil, mascarpone cheese, sautéed onion, and garlic, blending until creamy. Adjust consistency with reserved pasta water.

Return drained pasta to the pot, pour creamy spinach sauce over, and toss gently to coat. Adjust thickness with remaining pasta water if necessary.

Serve pasta in bowls, garnished with additional grated Parmigiano-Reggiano, fresh basil, and a drizzle of olive oil. Enjoy!