While heating the water and cooking the pasta, season the shrimp with salt and black pepper. In a 12 inch skillet over medium-high heat, heat the oil until shimmering. Add the shrimp in an even layer and cook without stirring until lightly browned on the bottoms, about 2 minutes. Transfer the shrimp to a bowl, leaving the oil in the pan.

Add the onion to the pan with a touch of salt, the cook, stirring occasionally, until beginning to brown, about 5 minutes. Add the garlic, oregano and pepper flakes. Cook, stirring until fragrant about 1minute. Add the tomatoes with juices and bring to a simmer. Cover, reduce to medium and cook for about 5 minutes.

Mix together the onion and tomatoes with the orzo and spinach. Taste and season with salt and black pepper. Transfer to a serving bowl, the sprinkle with the remaining feta and drizzle with additional oil.