Preheat the oven to 400 degrees. Place the tortillas on a baking pan and spray or rub lightly with olive oil. Bake for about 12-15 minutes or until golden and crispy. Remove from oven and set aside.
Meanwhile, heat about a tablespoon of olive oil in a skillet over high heat. Add the shishito peppers and onion; season with salt to taste.
Cook peppers and onions until the they soften and begin to get charred bits, stirring every few minutes. {About 5-7 minutes} Squeeze lime juice over the top and then cover with a lid and reduce the heat to low. Let them steam until tender, about 3-5 minutes.
While the peppers and onions cook, make the yogurt sauce: In a small bowl whisk together yogurt, salt, cumin and lime juice until combined. Set aside.
To build the tostadas: Spread about 2 tablespoons of refried beans on each tostada. Top each with the pepper & onion mixture. . Drizzle with yogurt sauce & add additional toppings like avocado slices queso fresco just before serving.