Heat the oven to 400 degrees F. Peel the sweet potatoes, slice a thin strip off one long side so each sits flat, then cut lengthwise into planks about ⅛ inch thick.

Lay the planks in a single layer on two oiled sheet pans and roast 15 minutes, until pliable and slightly dry at the edges. Set aside to cool.

Soften the onion, carrot and celery in the olive oil over medium heat for 8 minutes, then add the garlic and cook 1 minute more.

Raise the heat and brown the ground beef, breaking it apart, until it has real color.

Stir in the tomato paste and cook 1 minute until it darkens, then add the crushed tomatoes, beef broth, milk, oregano and bay leaves.

Simmer uncovered 30 minutes, until thick enough that a spoon dragged through leaves a track. Remove the bay leaves and season with salt and pepper.

Stir the ricotta with the egg, parmesan, parsley and black pepper until smooth and spreadable.

Drop the oven to 375 degrees F. Spread a thin layer of bolognese in a white ceramic baking dish, then layer overlapping sweet potato planks, a thin even layer of ricotta, and a spoonful of bolognese spread flat. Repeat for three full rounds, finishing with planks and sauce.

Cover with the mozzarella and parmesan.

Cover with foil and bake 35 minutes, then uncover and bake 20 minutes more until the top is browned and bubbling.

Rest the dish on the counter for 20 minutes before cutting so the squares hold their layers.