Heat oven to 425°F. Toss tomatoes with their oil and salt; roast cut-side up on rimmed pan 20 to 25 minutes until softened with browned edges. Cool 10 minutes.
Meanwhile, boil fusilli with tablespoon salt until fully tender. Reserve 1/2 cup pasta water; drain and spread in shallow dish 10 minutes.
Food-process basil, walnuts, Parmesan, garlic, pesto oil, lemon juice, pesto salt, and pepper finely. Blend in 2 tablespoons reserved water to spreadable pesto.
Toss pasta with pesto; add water by tablespoon only as needed to spread coating. Gently fold in cooled tomatoes.
Serve cool or cover and promptly refrigerate 30 minutes. Stir before serving; refrigerate leftovers within 2 hours.