Place your cashews in a heat proof bowl or small sauce pan. Boil enough water to more than cover your cashews and soak for at least 15 minutes, then drain and set aside.
Meanwhile, add the spinach to a large non stick skillet and wilt over medium heat. Add a drop or two of water to get it going, if needed. Stir often to prevent sticking. Once wilted, squeeze as much liquid out of the spinach as possible using a clean dish towel, nut milk bag or paper towels to do so.
Add the drained cashews, with all remaining ricotta ingredients EXCEPT the spinach to a food processor. Process until smooth, scraping down the sides as needed. Once smooth, add the spinach and pulse 2-3 times to get it incorporated. Do NOT over process the spinach.
Preheat the oven to 400 degrees F and line a baking sheet with parchment paper.
Brush your mushrooms clean and scrape out the gills using a metal teaspoon. Use caution to avoid breaking the mushrooms. Then place them stem side down on your tray. Sprinkle the tops with salt and pepper and bake for 10 minutes. This step will allow some moisture to escape the mushrooms so that your stuffed portobellos will not be soggy later.
While the mushrooms are baking, combine all the bolognese sauce ingredients into a small bowl. Mix and set aside.
Remove the mushrooms from the oven and flip them stem side up. Absorb any excess water that the mushrooms released with paper towels.
Divide the ricotta mixture between your six mushrooms and stuff them. Then top each one with 2-3 tablespoons of the bolognese sauce. Bake for another 12 minutes and remove from the oven and serve sprinkled with basil, parsley, chili flakes or vegan parmesan cheese if desired.
If using frozen spinach, measure out about 1 cup and thaw and squeeze out as much moisture as possible using a nut milk bag, dish cloth or paper towels.Do not over process the spinach when making the ricotta. Just pulse 2-3 times to get it mixed in. You want to see ribbons of spinach throughout your ricotta, and not green cheese.If planning ahead, you may make the ricotta cheese and the bolognese sauce and refrigerate the night before. Then stuff and bake your mushrooms the day of serving. Don't stuff them too far in advance.