1. Bring a large pot of water to a boil with 1 tablespoon salt. Add pasta, and cook according to package directions; drain.
2. Heat 1 tablespoon oil in a large nonstick skillet over medium-high heat. Add onion and oregano; saut 12 minutes or until lightly browned. Add garlic; saut 2 minutes. Add tomatoes and vinegar; saut 3 minutes or until tomatoes begin to soften. Add pasta and spinach; cook 1 minute. Remove from heat, and stir in Parmigiano-Reggiano, remaining 1 tablespoon oil, remaining 1/4 teaspoon salt, and pepper. Sprinkle with feta.
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