Step 1Cook pasta shells in plenty of boiling salted water until tender but still firm to the bite, 12 to 15 minutes or according to package instructions. Drain.
Step 2Chop spinach in food processor. Transfer to bowl and stir in Romano cheese, garlic, egg, 1/2 teaspoon salt and red pepper flakes to taste. Add ricotta and mix well.
Step 3Gently toss pasta shells in oil. Put 2 slightly rounded tablespoons of cheese-spinach mixture in each of 16 pasta shells. Close shells, slightly overlapping 1 side of each.
Step 4Spread 1 1/2 cups pasta sauce evenly in bottom of 13x9-inch shallow baking dish. Arrange shells, rounded side down, in single layer over sauce. Spoon remaining 1/2 cup sauce evenly over shells. Sprinkle mozzarella and Parmesan over sauce. Cover with foil. (If made ahead, bring to room temperature before baking.)
Step 5Bake covered at 350 degrees until sizzling, about 35 minutes. Sprinkle with basil and serve hot.