Heat 2 tablespoons oil in a large straight-sided skillet over medium-high heat.
Add sliced sausage, 1 cup onion and minced garlic; cook, stirring often, until the onion is starting to brown, 4 to 6 minutes.
Add 1 (8-ounce) can tomato sauce, 4 cups spinach, 6 cups cooked pasta, ¼ cup olives, ¼ teaspoon salt and ¼ teaspoon crushed red pepper; cook, stirring often, until the mixture is bubbling and the spinach is wilted, 3 to 5 minutes.
Add ¼ cup water, or more as needed, to reach desired consistency.
Stir in ½ cup feta and ¼ cup basil.
