In a large skillet, heat olive oil over medium heat. Saute onions and red peppers until onions are translucent, about 5 minutes. Toss in the cumin, paprika, and garlic. Simmer spices until fragrant, about 1-2 minutes. Stir in the tomatoes, vinegar, and salt. Allow sauce reduce on low for 15-20 minutes. Mix in the feta cheese and remove the skillet from heat.