To thaw shrimp, place frozen shrimp in a colander and run cold water over them for 5-10 minutes until thawed.
In a medium pan, bring water to a boil for the pasta. Meanwhile in a small sauce pan start making the sauce.
To make the sauce, heat the olive oil over medium heat. Add garlic and cook for around a minute - don't let the garlic burn! Add flour and cook for around 30 seconds stirring the entire time to avoid burning. Add the milk and stir constantly making sure all the flour mixture is well incorporated and not stuck to the bottom of the pan. Bring to a boil. Boil for 2-3 minutes or until starts to thicken. Stir throughout to avoid sticking and burning. Remove from heat. Whisk in Greek yogurt, Parmesan cheese, and salt. Set aside.
Throw your pasta into the boiling water to cook. Meanwhile, cook your spinach and shrimp.
In a skillet, heat olive oil over medium heat. Add garlic and spinach. Cook until spinach is wilted. Push spinach to the side of the pan. Add shrimp and a little salt and pepper. Cook the shrimp until pink.
Drain your pasta and place it back in the medium saucepan. Add sauce and spinach/shrimp mixture. Note: if there is some water in the spinach pan, scoop out the spinach and shrimp. Try not to add the excess water. Combine with tongs.
Pour into a baking dish (9x9 or 11x7 or pie pan) and top with extra cheese if you want.
Bake in a 350 degree oven for 10 minutes. Broil to brown the top if you want to get crazy. Note: don't broil glass pans!