Bring 4 cups water, lemon juice, 1 1/4 teaspoons salt, 1/2 teaspoon pepper and crushed red pepper if using to a boil in a large saucepan.
Stir in the oats, spinach and tomatoes, reduce the heat to medium and cook, stirring frequently, until the oats are just softened and the mixture is slightly thickened, about 7 minutes. Remove the saucepan from the heat and stir in the feta, oil and dill.
Transfer the oatmeal to individual bowls and garnish with scallions and olives if using.
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