This recipe is inspired by an Indian vegetarian dish called saag paneer; here, the traditional soft, unripened cheese is replaced with tofu.
Heat oil in a large skillet over medium-high heat. Add onion, garlic, 2 teaspoons salt, and 1/4 teaspoon pepper. Cook, stirring occasionally, until onion is beginning to brown, 5 to 7 minutes. Add curry powder and ginger; cook, stirring, until fragrant, about 1 minute.
Add spinach and 1 1/2 cups water; bring to a boil, breaking up spinach with a spoon. Reduce heat to medium; simmer, stirring occasionally, until almost all liquid has evaporated, 10 to 12 minutes.
Stir in sour cream and tofu; cook just to heat through, 3 to 5 minutes (do not boil). Serve with rice, if desired.
In this recipe, the frozen spinach is cut into chunks; this will allow it to thaw quickly while cooking.