In a large saucepan, bring 4 cups water, lemon juice, 1 1/4 teaspoon salt, 1/2 teaspoon pepper, and crushed red pepper, if using, to a boil.
Reduce the heat to medium, whisk in the oats, spinach, and tomatoes, and cook, stirring regularly, for about 7 minutes, or until the oats are just softened and the mixture is somewhat thickened. Remove the skillet from the heat and add the feta, oil, and dill to it.
Serve the oats in separate dishes with scallions and olives, if desired.