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 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. Divide the pepper and onion mixture between all five tostadas. Just before serving, drizzle with yogurt sauce and add additional toppings like avocado slices and queso fresco if desired. Serve immediately.
