Heat 2 tablespoons oil in a large frying over medium-high heat. When it shimmers, add onion, season well with salt and freshly ground black pepper, and cook, stirring occasionally, until soft and translucent, about 5 to 6 minutes. Remove from heat, stir in half of herbs, and set aside.
Return pot to stove and add in onion mixture, remaining 1/4 cup oil, remaining herbs, reserved pasta water, and zest, and season well with salt and pepper. Stir until evenly combined. Add pasta and toss until pasta is well coated. Taste and adjust seasoning, as necessary.