Bring a large pot of water to a rolling boil over high heat. Add fresh spinach and blanch for 30 seconds until vibrant green; immediately transfer to an ice bath then drain well.

In a medium skillet, heat 2 tablespoons of extra virgin olive oil over medium heat. Add diced yellow onion and sauté for 10 to 12 minutes until soft. Stir in minced garlic and cook for an additional minute, then remove from heat.

In a large pot, bring salted water to a boil and add tagliatelle. Cook until al dente, typically 8 to 10 minutes. Reserve 1 cup of pasta cooking water before draining.

In a blender, combine the blanched spinach, fresh basil, parsley, pistachios, and lemon juice with 2 tablespoons of olive oil. Blend until finely minced, then add the sautéed onions, garlic, and mascarpone cheese. Blend until smooth, incorporating half the grated Parmigiano-Reggiano.

In the large pot, return cooked tagliatelle and pour in the creamy spinach pasta sauce. Toss gently, adding reserved pasta water to achieve desired consistency.

Divide the pasta into bowls, garnishing with remaining Parmigiano-Reggiano and fresh basil. Drizzle olive oil on top.