In a large skillet, heat 2 tablespoons of vegan butter or olive oil over medium heat. Add 4 sliced sweet onions and sauté for 25-30 minutes, stirring occasionally until they turn a rich, golden brown. Towards the end of cooking, drizzle in 1 tablespoon of agave syrup and add a pinch of salt, stirring well.

While the onions caramelize, blend 14 ounces of drained extra firm tofu in a food processor. Add 1/4 cup of nutritional yeast, 2 tablespoons of lemon juice, and 1/3 cup of non-dairy milk, processing until smooth and creamy. Add more non-dairy milk if too thick.

In a separate saucepan, melt 3 tablespoons of vegan butter over medium heat. Whisk in 1/4 cup of flour to create a roux. Gradually whisk in 2 cups of non-dairy milk, cooking for 2-4 minutes until the sauce thickens. Stir in 1 tablespoon of fresh thyme and a pinch of nutmeg.

Preheat your oven to 375°F (190°C). In a 9x13-inch baking dish, spread a layer of béchamel on the bottom. Arrange lasagna sheets over the sauce, followed by dollops of tofu ricotta, a layer of caramelized onions, and a sprinkle of vegan mozzarella. Repeat layers, finishing with béchamel and a topping of vegan cheese.

Cover the baking dish with aluminum foil and place it in the preheated oven. Bake for 25-30 minutes, removing the foil in the last 10 minutes. Let it cool for about 10 minutes before slicing.