Cook your shells per manufacturer''s instructions, making sure to salt your pasta water. Drain through a colander and set aside until you are ready to stuff them.
For the seafood filling, heat 1 tablespoon of olive oil in a small non-stick skillet. Add the shallot, green and red bell peppers. Season with salt and pepper and cook until tender, approximately 4-5 minutes. Remove from heat and set aside to cool slightly.
In a large bowl, combine the shallot and pepper mixture, mozzarella cheese, parmesan cheese, mayo, whole milk, egg, panko bread crumbs, lump crab meat, chopped shrimp, seafood seasoning and a dash of salt and pepper. Stir together thoroughly. Set aside.
Preheat your oven to 350 degrees F and grease a 9x13in baking dish. Set aside.
Stuff each shell with a tablespoon of the seafood filling. Place in the 9x13in baking dish. Repeat this step for the rest of the shells. You should be able to fit about 20-24 shells in the baking dish. Once you have stuffed all of your shells, set aside.
To prepare your sauce, melt 6 tablespoons of butter over medium heat. Add the yellow onion and saute for 4-5 minutes or until tender. Add the garlic, salt and pepper and cook for another minute. Very slowly, add the flour into the pan. Whisk until the butter and onion have absorbed the flour and all of the rawness from the flour is cooked out, approximately 3-4 minutes. While you continue to whisk, very slowly pour in the heavy cream and the whole milk. Keep whisking until all of the ingredients have been incorporate, approximately 2-3 minutes. Add in the seafood seasoning and more salt and pepper to taste. Let cook for 10-15 minutes or until the sauce has thickened substantially. Add the fontina cheese and stir to melt. Pour the sauce over the shells, reserving about a cup for later.
Bake the shells, uncovered, for 25-30 minutes or until bubbly. Remove from oven and garnish with parsley. Serve with extra sauce. Enjoy!