Brown the meat: Heat a drizzle of olive oil in a skillet over medium. Add ground beef and onion. Cook until the meat is browned and onion is soft, 6–8 minutes. Drain excess fat if needed.

Add aromatics and season: Stir in garlic, basil, oregano, red pepper flakes, a pinch of salt, and a few grinds of black pepper. Cook 1 minute until fragrant.

Make the sauce: Pour in marinara and the canned tomatoes (if using). Simmer 3–5 minutes to marry flavors. Taste and adjust salt and pepper.

Mix the ricotta layer: In a bowl, combine ricotta, egg, Parmesan, a pinch of salt, and some pepper. Stir until smooth.

Prep the slow cooker: Spray the insert with nonstick spray or wipe with oil. If you want easy removal, lay a wide strip of parchment or a foil sling across the bottom and up the sides.

Start layering: Spread a thin layer of sauce on the bottom—just enough to coat. Break noodles to fit into a single, even layer.

Add cheeses: Dollop and spread about one-third of the ricotta mixture over the noodles. Sprinkle a handful of mozzarella on top.

Repeat layers: Continue layering in this order—sauce, noodles, ricotta, mozzarella—until you’ve used everything. Aim for 3–4 layers. Finish with a generous layer of sauce and the remaining mozzarella and a little Parmesan.

Cook: Cover and cook on Low for 4–5 hours or High for 2.5–3.5 hours, until the noodles are tender and the edges are bubbly. Avoid opening the lid too often.

Rest and slice: Turn off heat and let the lasagna rest, covered, for 20–30 minutes. This helps it set so it slices cleanly.

Garnish and serve: Sprinkle with extra Parmesan and chopped basil or parsley. Slice into squares and serve warm.