Preheat the oven to 375°F (190°C). Lightly oil a 9x13-inch baking dish.

Heat 1–2 tablespoons of olive oil in a skillet over medium heat. Add the chopped onion and sauté until translucent, about 4–5 minutes. Stir in the minced garlic and cook for another 30–60 seconds until fragrant.

Add the vegan sausage (crumbled or sliced) to the skillet, cooking until lightly browned, about 5–7 minutes. Season with a pinch of salt and pepper, then remove from heat.

In a medium bowl, combine the vegan ricotta with the spinach. If using raw spinach, wilt quickly in the skillet first and squeeze out excess moisture before mixing. Season the ricotta-spinach mix with salt and pepper to taste.

Spread a thin layer of marinara on the bottom of the prepared baking dish. Lay down the first layer of noodles (trim to fit if needed), then spoon a layer of marinara over the noodles.

Scatter some of the sausage mixture and dollop the ricotta-spinach mixture across. Repeat layers (noodles, sauce, sausage, ricotta) until you reach the top of the dish. Finish with a generous layer of marinara.

Sprinkle nutritional yeast evenly over the top for a cheesy finish. Cover the dish tightly with foil and bake for 30 minutes.

Remove the foil and bake for an additional 15–20 minutes until the edges are bubbling and the top is slightly golden.

Let the lasagna rest for 10–15 minutes before slicing to help it set and keep slices intact.