Preheat oven to 400°F and line two baking sheets with parchment paper or foil.
Arrange tortillas in a single layer, brush lightly with avocado oil, and sprinkle with kosher salt.
Bake for 10–12 minutes, flipping halfway through, until crispy and golden brown.
Heat a skillet over medium heat with a drizzle of oil and cook the corn for 4–5 minutes until lightly charred.
Transfer the corn to a mixing bowl and stir in red onion, cilantro, mayonnaise, lime juice, chili-lime seasoning, and cotija cheese.
Warm the refried black beans until spreadable.
Spread beans over each tostada shell and top generously with the street corn mixture.
Finish with extra cotija cheese, additional chili-lime seasoning, and serve with lime wedges.
