In a food processor, combine fresh basil, parsley, olive oil, sunflower seeds, grated Pecorino Romano cheese, lemon juice, crushed garlic, and sea salt. Blend until desired texture is achieved.

Reserve about four tablespoons of pesto for later use.

Fill a large pot with lightly salted water and bring to a boil. Cook the tagliatelle until tender yet firm, around 9 to 10 minutes.

In a large skillet, heat olive oil over medium heat. Add minced garlic and red pepper flakes, cooking for about one minute.

Add diced onion, salt, and cracked black pepper. Sauté until onions are soft and translucent, about four minutes.

Drain pasta, reserving around a quarter cup of pasta water. Transfer drained tagliatelle to the skillet.

Add the reserved four tablespoons of pesto and the reserved pasta water to the skillet. Toss to combine, ensuring pasta is well coated.

Remove skillet from heat; add additional Pecorino Romano and lemon juice. Adjust seasoning as needed.

Plate the tagliatelle and garnish with extra cheese, basil leaves, or sunflower seeds as desired.