Preheat oven to 400°F (200°C).
Lightly brush both sides of the corn tortillas with avocado oil and place them on a baking sheet.
Bake tortillas for 4-5 minutes per side until they are golden brown and crisp.
While tortillas bake, place the rinsed black beans in a small saucepan over medium heat with the corn kernels, chili powder, cumin, and garlic powder.
Add a splash of water to the beans and use a fork to mash about half of them, then stir in the nutritional yeast until well combined and heated through.
In a small bowl, whisk together the non-fat Greek yogurt, lime juice, and sea salt to create a high-protein crema.
To assemble, spread the warm black bean mash evenly over the crispy tostada shells.
Top with shredded romaine lettuce, diced avocado, and minced red onion.
Drizzle the lime-yogurt crema over the top and serve immediately.