Heat the oven to 425F. Prick the sweet potatoes all over and roast directly on the rack for 45 to 60 minutes, until a knife slides into the thickest part with no resistance.

Split them open, scoop the flesh into a bowl, and let it sit uncovered for 10 minutes so the steam escapes. Mash with a fork, leaving a little texture, and cool completely.

Stir the ricotta, egg, ¾ cup parmesan, garlic, nutmeg, salt, and pepper into the cooled mashed sweet potato until evenly combined. Taste and adjust the salt.

Boil the shells in well-salted water for 2 minutes less than the package directs. Drain and spread them in a single layer on a sheet pan so they do not stick together.

Spoon the filling into a large zip-top bag, cut a ¾-inch opening from one corner, and pipe about 2 tablespoons into each shell. Do not overfill.

Melt the butter in a saucepan over medium heat, whisk in the flour, and cook for 1 minute until it smells nutty.

Pour in the warm milk in a steady stream while whisking. Cook 5 minutes, until the sauce coats the back of a spoon.

Off the heat, stir in the sage, ¼ cup parmesan, salt, and pepper.

Heat the oven to 375F. Spread about 1 cup of sauce over the bottom of a 9x13-inch baking dish.

Arrange the filled shells ridged side up in the dish, close together but not stacked. Spoon the remaining sauce over them and scatter the last ¼ cup parmesan across the top.

Cover with foil and bake 25 minutes. Uncover and bake 10 to 15 minutes more, until the top is spotted golden and the edges bubble.

Rest 10 minutes before serving so the sauce thickens.