Preheat oven to 400 degrees Fahrenheit. Line a baking sheet with parchment paper and spread corn tortillas out (they will be overlapping slightly). Coat both sides of the corn tortillas with avocado oil (or olive oil/vegetable oil spray).

Bake in the oven for 10 minutes total, flipping the tortillas over after 5 minutes.

While the tostadas are cooking, in a medium-sized pan, heat olive oil over medium heat. Add mushrooms and onion and sauté for about 5 minutes, until mushrooms are cooked through. Note: if desired, reserve a few sautéed mushrooms to top tostadas with at the end (it looks nice when serving to adults!).

Empty the can of refried beans into a food processor or high-speed blender and add coconut aminos. Then, transfer the mushrooms and onions to the food processor. Blend until smooth.

Spread bean and mushroom mixture onto the tops of each tostada. Serve with any desired toppings such as red onion, cilantro, salsa, plain yogurt/sour cream, lime, and guacamole.