Preheat oven to 375 degrees.
Make broccoli pesto. Simply combine all ingredients in a blender or food processor and pulse until a smooth blend forms. Taste for salt and pepper as needed. Pour into a covered jar or food storage container and set aside.
Heat butter over medium in a small saucepan. Whisk flour into butter until no chunks remain, then slowly pour in milk, whisking constantly. Bring to a gentle boil and whisk until the sauce thickens, then add cheese and garlic powder. Mix until cheese melts, and add salt and pepper as desired. Remove from heat.
To a large casserole dish, add broccoli pesto, oregano, spinach, water, and bechamel sauce. Swirl with a spoon until combined, then pour in pasta and spread evenly across the casserole dish. Press pasta into liquid to submerge. Dot top of the pasta with mozzarella cheese.
Cover dish tightly with foil and bake in preheated oven for 40 minutes - pasta will be slightly firmer than al dente at this point. Remove foil and bake an additional 10 minutes or until pasta is tender to your liking. You might need an additional 2-5 minutes if you prefer a softer texture. Turn on broiler for 2-3 minutes until mozzarella browns slightly. Remove from oven and let cool 5 minutes. Serve and enjoy!