Start by preparing your sweetpotatoes. Preheat oven to 425 degrees and line a baking tray with parchment paper.
Pierce the two sweetpotatoes all over with a fork. Place on a parchment-lined baking tray and cook for 45-50 minutes, or until you can easily insert a knife through the thickest part of the sweetpotato.
Remove from the oven and let cool. Once cool, remove skins and move to a small bowl. Mash using a fork or potato maker.
While sweetpoatoes are cooking, boil your shells until al dente, 8-9 minutes (shells will continue to cook through when baked in the oven.)
For your filling, measure 1 1/2 cups of mashed roasted sweetpotato and mix together with ricotta, egg, salt, sage and black pepper until well combined. Set aside.
Start on your sauce - in a medium cast-iron skillet, add 1/2 stick butter and brown over medium heat.
Once butter is browned, turn heat to medium low, and add two tablespoons of flour, whisking continuously to ensure flour does not burn (flour should be sizzling). Cook for 3-5 minutes or until flour turns a light brown color.
Add diced onions, and using a wooden spoon, stir and cook for an additional 3-5 minutes or until onions start to brown.
Stir in one tablespoon Better Than Bouillon, organic vegetable base. If you don't have, leave out and add one teaspoon salt instead. Stir until evenly combined.
Slowly whisk in one pint heavy whipping cream. Bring to a simmer and remove from heat, set aside. To a small bowl, add 1/4 of the sauce and set aside for later.
Start preparing your shells - split filling evenly between 20 shells and place directly in the cast-iron skillet with sauce (you might need to squish in the shells!).
Finish by topping off the extra sauce, ground parmesan, and fresh mozzarella.
Bake at 350 degrees for 25 minutes, broiling for the last 2-3 minutes to brown mozzarella cheese if needed.
Optional: remove and top with fried sage leaves (brown butter in a small saute pan and add fresh sage leaves cooking for one minute or until crisp). Enjoy!