Heat up a large skillet over medium flame. Add a splash of water and sautée the onion with a pinch of salt until translucent, about 5 minutes or so. Add the mushrooms and cook together another 10 minutes or so until wilted and the water has evaporated. Taste and season with sea salt. Set aside until needed.
In the same skillet add another teaspoon of water and add the spinach. Sautee until wilted then season with a tiny pinch of sea salt. Transfer to a colander to drain. Once cool enough to handle use your hands to squeeze as much of the liquid as possible. Set aside.
Brig out your lasagna pan ( 8X12 works fine) and spread 1.5 cups of the tomato sauce on the bottom. Layer enough of the lasagna noodles to slightly overlap each other and cover the bottom. You might need to break a couple of them to fill in any gaps on the sides depending on the shape of your pan.
Add the cashew ricotta over the top and spread it across in an even layer. (Sprinkle with a pinch of freshly grated nutmeg at this point if desired).
Top with the sautéed spinach and mushrooms then add 1.25 cups of the tomato sauce.
Add another layer of the lasagna noodles slightly overlapping each other and finish with a layer of the remaining sauce.
Finish with a layer of the vegan smoked gouda on top and the sliced heirloom tomatoes. Sprinkle with a pinch of sea salt and red pepper flakes.
Cover the lasagna pan with aluminum foil ( or a lid) and transfer it to the preheated oven for 45 - 50 minutes until the noodles have cooked through.
Allow to cool and set for a good 15 minutes before serving with the fresh basil on top.