In a large, oven-safe skillet add olive oil and place over medium heat.
Once hot, add onions and cook for 3 minutes until starting to soften.
Add garlic, oregano and red pepper flakes. Cook for another 2 minutes until fragrant.
Add diced tomatoes (with the juices from the can), spinach and olives. Season with salt and pepper. Stir to combine and simmer over medium-low heat for 5-7 minutes until slightly reduced and thickened.
Arrange shrimp on top of tomato mixture. Crumble feta cheese around skillet and squeeze lemon juice on top.
Place the skillet in the oven and bake for 12-15 minutes until shrimp are pink.
Switch the oven to broil and broil for 1-2 minutes until feta starts to turn golden brown.
Remove from oven, garnish with fresh oregano and serve with crusty bread or rice.