Heat the oil in a large skillet over medium-high heat. Add the onions and cook, stirring occasionally, until softened and just starting to turn brown in some spots, 4 to 5 minutes. Stir in the garlic, Italian seasoning, 1/2 teaspoon salt and a few grinds of black pepper, and stir until the garlic is softened, about 1 minute.
Pour in the tomatoes and bring to a boil. Reduce the heat to medium and simmer, stirring occasionally, until the tomatoes are slightly thickened, 2 to 3 minutes. Add the shrimp and beans and cook, stirring occasionally, until the shrimp just start to turn orange, 2 to 3 minutes. Stir in the spinach and continue cooking, stirring occasionally, until the spinach is just wilted and bright green and the shrimp are cooked through and opaque, 2 to 3 minutes. Remove the skillet from the heat and sprinkle the feta over top. Cover until the cheese is warmed through, 2 to 3 minutes. Serve immediately.
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