Bring a large pot of water to a rolling boil. Blanch spinach for 30 seconds, transfer to ice bath, then drain.

In a pot, heat 1 tablespoon olive oil over medium heat. Sauté onion for 10-12 minutes, then add garlic for 1 minute.

In the same pot, boil water, add salt, and cook tagliatelle until al dente, reserving 1 cup pasta water.

Blend spinach, basil, parsley, pistachios, lemon juice, and remaining olive oil until minced. Add mascarpone, onion, garlic, and half Parmigiano-Reggiano, blending until smooth.

Toss tagliatelle with the sauce, using reserved pasta water for consistency.

Serve in bowls, garnishing with remaining Parmigiano-Reggiano and olive oil.