Prep all ingredients and hold for use. This is called "mise en place" and it will make cooking MUCH easier for you.
Method
Fill large pot 2/3 with water for pasta and set on high heat to boil.
Melt butter and olive oil in skillet over medium high heat. When melted, add garlic and shallots. Turn down heat to medium and sweat (do not brown) until translucent.
Add in scallops and shrimp to shallots and gently cook until they are opaque. 3-4 minutes. Hold.
While seafood cooks, add spinach and parmesan to food processor. Pulse 2-3 times to break up spinach and incorporate parmesan.
Season pasta water (which should be at the boil by now) with 2 Tbl of salt and add shells. Moderate heat to a boil, but not boiling over. Cook pasta until al dente - use package instructions for time.
Add seafood and all pan drippings to spinach in food processor and pulse until incorporated, but NOT a paste. You want to have chunks of seafood here. Add ricotta and nutmeg and pulse once or twice to combine. Check seasoning.
Check shells for doneness and when finished, drain and rinse with cold water.
Put mixture from processor into a large plastic bag. Cut off one corner (about ¾ inch from tip) to make a piping bag. Spray baking dish with food release spray and put a few swirls of olive oil in the bottom. Pipe filling into shells and place into baking dish. Set aside. Preheat oven to 375°F.
This is a bechamel sauce. Melt sauce butter in a sauce pan over medium high heat. When butter is bubbly, add flour and stir for a minute or so to make roux. Add half and half and bring to boil, stirring constantly to incorporate roux. Turn down heat to medium and add parmesan, stirring constantly (use a whisk if you have one). Add nutmeg. You should have a thick, cheesy sauce. If it's too thick, add a little water. If it's too thin, add more parmesan. When you've got the consistency right, turn off heat and check seasoning.
Pour bechamel sauce over pasta and bake at 375°F for about 20 minutes or until the sauce is bubbly and has browned a little. Let rest 5 minutes before you serve.