Prep the oven and pan. Heat the oven to 375°F (190°C). Grease a 9x13-inch baking dish with olive oil or butter.

Slice the zucchini. Use a mandoline or sharp knife to slice lengthwise into thin planks, about 1/8 inch thick. Aim for even slices so they cook uniformly.

Reduce moisture from zucchini. Lay slices on paper towels, sprinkle with a little salt, and let sit for 10–15 minutes. Pat dry. For extra insurance, briefly grill or pan-sear slices 1–2 minutes per side to release water.

Cook the sausage. In a large skillet over medium heat, cook the sausage until browned, breaking it into crumbles. Remove with a slotted spoon and set aside.

Sauté aromatics and veggies. In the same skillet, add a bit of fat if needed. Sauté onion and bell pepper until softened, about 4–5 minutes. Stir in garlic for 30 seconds. Add spinach and cook until wilted. Season with salt, pepper, and Italian seasoning.

Make the ricotta layer. In a bowl, mix ricotta (or well-drained cottage cheese) with half the Parmesan, a pinch of salt, and black pepper. Set aside.

Whisk the egg mixture. In another bowl, whisk eggs with heavy cream, a pinch of salt and pepper, and crushed red pepper if using.

Assemble the first layer. Arrange a single layer of zucchini slices on the bottom of the baking dish. Don’t overlap too much; slight gaps are fine.

Add savory layers. Scatter half the sausage and half the sautéed veggies over the zucchini. Dollop and spread half the ricotta mixture on top. Sprinkle with a handful of shredded cheese.

Repeat. Add another layer of zucchini, the remaining sausage and veggies, the rest of the ricotta, and another handful of shredded cheese.

Finish with eggs. Pour the egg-and-cream mixture evenly over the top, letting it seep into the layers. Top with the remaining shredded cheese and the rest of the Parmesan.

Bake. Cover loosely with foil and bake for 20 minutes. Remove foil and bake another 15–20 minutes, until the center is set and the top is golden and bubbly.

Rest and garnish. Let the lasagna rest for 10–15 minutes to firm up before slicing. Garnish with chopped basil or parsley.

Slice and serve. Cut into squares and serve warm. A spoonful of sugar-free marinara on the side is optional but tasty.