In a medium saucepan, heat serrano, mushrooms, onion, vegetable broth, lime juice and optional canola oil (see notes for oil-free) over medium-high heat. Cover and allow to boil for 3 minutes.
After 3 minutes, remove the cover and continue cooking until all the liquid has evaporated and the mushrooms begin to sizzle.
Add the tomatoes and cilantro. Stir often until the liquid has reduced and the mixture is thick, about 10 minutes. Remove from heat.
Avocado Cream
While the mushrooms stew, make the avocado cream. Pour the salsa verde into a blender or food processor. Add the avocado flesh. Blend until nearly smooth.
Pour into a dish and slowly mix in the water until the avocado cream thins to a drizzling consistency.
Assemble Tostadas
Set the tostadas on a plate and drizzle with the avocado sauce. Scoop 1/4 cup of the mushroom mixture onto each one of the tortillas. If desired, garnish each tostada with fresh cilantro and vegan parmesan. Enjoy!