Place your spinach in 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. Set aside.
Add the ricotta cheese ingredients EXCEPT the spinach to a food processor. Process until smooth, scraping down the sides as needed. Once smooth, add the spinach and pulse 3-4 times to get it incorporated. Do NOT over process the spinach or the cheese will turn green.
Preheat the oven to 400 degrees F and line a large 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 allows some moisture to escape the mushrooms so that they won't be watery 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 the excess moisture using a clean dish towel or paper towels.
Divide the ricotta between your six mushrooms. Then top each one with 2-3 tablespoons of the bolognese sauce. Bake for another 10-12 minutes and remove from the oven and serve sprinkled with basil, parsley, chili flakes or vegan parmesan cheese if desired.
If you don't have a powerful food processor, soak the cashews in boiling hot water for 15-30 minutes, and then drain, before making your vegan ricotta cheese.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 a few times to get it mixed in. You want to see ribbons of spinach throughout your ricotta, and not green cheese.To make these into appetizers, use about 2-3 times as many button or cremini mushrooms instead, depending on their size.If planning ahead, you may make the ricotta cheese and the bolognese sauce and refrigerate up to 2 days before. Then stuff and bake your mushrooms the day of serving. Don't stuff them too far in advance.Your leftover vegan stuffed portobello mushrooms keep refrigerated 2-3 days in a sealed container.