Preheat the oven to 425 degrees Fahrenheit (220 degrees Celsius). Place the mushrooms and corn on a large-rimmed baking sheet and drizzle with the olive oil. Sprinkle the salt, cumin, and garlic powder over the top, then use a wooden spoon to toss the vegetables and coat with oil. Arrange the vegetables in a single layer on the pan (as best you can). Roast for 15 minutes, stirring once halfway through.
Meanwhile, pour about 2 inches of vegetable oil into a large pot and place over medium heat. Once the oil is shimmering, add the tortillas one at a time and fry until golden brown and crispy, about 1 1/2 minutes. Use tongs to carefully transfer the fried tortillas to a paper towel-lined baking sheet.
Assemble the tostadas by placing two fried tortillas on a plate, topping with a quarter of the mushroom-corn mixture, and topping with tomatoes, avocado, cilantro, and lime juice. Repeat with remaining tortillas.
Notes
To store Vegan Tostadas with Mushrooms and Corn leftovers, place the mushroom-corn mixture in an airtight container and refrigerate it in the fridge for up to 3 days. The fried tortillas are best eaten fresh but can be stored separately in a sealed container at room temperature for 1-2 days. It's not recommended to freeze the tortillas or the vegetable mixture, as they may lose texture.To reheat the mushroom-corn mixture, warm it in a skillet over medium heat until heated through.