Step 1

Arrange a rack in the middle of the oven and heat the oven to 400°F.

Step 2

Peel 2 pounds sweet potatoes. Cut crosswise into 1/8 inch-thick rounds with a mandoline. (Alternatively, cut a sliver of potato from one side to make a flat surface, then slice crosswise into 1/8 inch-thick slices with a knife.) Finely chop 1 large yellow onion and mince 3 cloves garlic. Pick the leaves from the 2 large fresh oregano sprigs and coarsely chop (2 tablespoons).

Step 3

Heat 1 tablespoon of the olive oil in a large skillet over medium heat until shimmering. Add the onion and cook, stirring frequently, until soft, about 8 minutes. Add the garlic and cook until fragrant, about 1 minute.

Step 4

Push the onion mixture to one side of the skillet and increase the heat to medium-high. Crumble 8 ounces ground beef and 8 ounces sausage into the empty side of the skillet, and break up the meat with a wooden spoon into smaller pieces. Cook until meat is browned underneath, then stir with onion mixture to combine. Season with 1 teaspoon of the kosher salt and 1/2 teaspoon of the black pepper.

Step 5

Add 3 tablespoons tomato paste and cook until slightly darkened in color, 1 to 2 minutes. Add 1 tablespoon of the oregano, 1 cup dry red wine, 1/4 cup Worcestershire sauce, 1 1/2 teaspoons Italian seasoning, and 1/4 teaspoon red pepper flakes, if using. Scrape up any browned bits from the bottom of the pan. Bring to a simmer, then cook for 1 to 2 minutes more to allow some of the alcohol to burn off.

Step 6

Add 1 (28-ounce) can crushed tomatoes. Add 1 cup water to the tomato can and swirl to release any leftover tomato sauce from the sides of the can. Add the tomato-water and 1 1/2 teaspoons granulated sugar to the pan and stir to combine. Bring to a simmer.

Step 7

Reduce heat to medium-low and simmer until sauce has thickened slightly and flavors have melded, 15 to 20 minutes. Meanwhile, finely grate 5 ounces Parmesan cheese (about 1 1/4 cups). Shred 16 ounces mozzarella cheese (about 4 cups).

Step 8

Make the ricotta filling: Finely zest 1 small lemon into a medium bowl. Thinly slice the leaves from 1 large fresh basil sprig and add to bowl. Add 2 large eggs and beat lightly with a fork to combine. Add 1 container ricotta cheese, 1/2 cup of the Parmesan, 2 tablespoons of the olive oil, the remaining 1 teaspoon kosher salt, and the remaining 1/2 teaspoon black pepper, and stir to combine.

Step 9

When the sauce is ready, taste and season with more kosher salt as needed. Coat a 9x13-inch baking dish with the remaining 1 tablespoon olive oil.

Step 10

Assemble the lasagna: Set aside the largest pieces of sweet potato for the top layer. Arrange 1/3 of the sweet potato slices in the baking dish in an even layer, overlapping them slightly as needed. Dollop half of the ricotta mixture onto the sweet potatoes and spread into an even layer. Spoon half the meat sauce over the ricotta and spread into an even layer. Sprinkle with 1 1/3 cups of the mozzarella.

Step 11

Layer with 1/2 of the remaining sweet potato slices, the remaining ricotta mixture, remaining meat sauce, and 1 1/3 cups of the mozzarella. Top with the remaining sweet potato slices, then sprinkle with the remaining 1 1/3 cups mozzrella and remaining 3/4 cup Parmesan.

Step 12

Place the baking dish on a rimmed baking sheet. Cover the baking dish loosely with aluminum foil. Bake until the sweet potatoes are almost cooked through, about 45 minutes.

Step 13

Uncover the lasagna. If needed, spoon any excess liquid off the top of the lasagna. Bake uncovered until the cheese on top is bubbly and golden-brown, about 15 minutes more. Let cool on a wire rack for at least 15 minutes before serving. Sprinkle with the remaining 1 tablespoon oregano before serving.