Preheat oven to 425 degrees. In an oven-proof dish, add olive oil, cherry tomatoes, salt, pepper, onion, and garlic. Mix to combine.
Make a space large enough for the feta block in the middle of the tomatoes, place feta into the dish and bake for 40-50 minutes until feta is browned and tomatoes are bubbling.
While the tomatoes and cheese bake, bring a pot of water to a boil and cook the pasta according to package instructions. Reserve ½ cup of the pasta water to add into your sauce later.
Remove the dish from the oven and add spinach, mix to combine, and allow the spinach to wilt; the feta should be soft and smooth. Add cooked pasta, grated Parmesan, and a splash of pasta water to the dish, mixing again to combine. Serve warm; can be stored in the fridge for up to three days.