Prep the zucchini “noodles.” Use a mandoline or sharp knife to slice zucchini lengthwise into thin planks, about 1/8 inch thick. Lay the slices on paper towels, sprinkle lightly with salt, and let sit 10–15 minutes to draw out moisture. Pat very dry. This step helps prevent a watery lasagna.

Brown the meat. In a large skillet over medium heat, warm the olive oil. Add onion and cook 3–4 minutes until softened. Stir in garlic for 30 seconds. Add ground beef or turkey, season with a pinch of salt and pepper, and cook until browned, breaking it up with a spoon. Drain excess fat if needed.

Build the sauce. Stir 2 1/2 cups of the marinara into the meat, plus oregano, basil, red pepper flakes, remaining salt, and black pepper. Simmer 3–5 minutes to meld flavors. Taste and adjust seasoning.

Mix the ricotta layer. In a bowl, combine ricotta, 1/4 cup Parmesan, and the egg. Stir until smooth. Season with a small pinch of salt and pepper.

Prep the slow cooker. Lightly coat the insert with cooking spray. Spoon a thin layer (about 1/2 cup) of the remaining marinara across the bottom to prevent sticking.

First layer. Arrange a single layer of zucchini slices on the bottom, slightly overlapping. Spread a third of the ricotta mixture over the zucchini. Spoon a third of the meat sauce over the ricotta. Sprinkle with a third of the mozzarella.

Repeat layers. Make two more layers in the same order: zucchini, ricotta, meat sauce, mozzarella. Finish with the remaining mozzarella and the remaining 1/4 cup Parmesan on top.

Cook low and slow. Cover and cook on Low for 4–5 hours or on High for 2 1/2–3 hours, until the zucchini is tender and the edges are bubbling.

Rest before slicing. Turn off the heat and let the lasagna rest, covered, for 20–30 minutes. This helps it set, so slices hold together.

Garnish and serve. Scatter with fresh basil or parsley. Slice with a spatula and serve warm.