1. Preheat the oven to 400. Lay tortillas on two rimmed baking sheets and toast in oven until crisp, about 5 minutes. Remove from baking sheets.
2. Lightly coat baking sheets with cooking spray. Spread beans and onion on one and zucchini and jalapenos on the other. Lightly coat vegetables with cooking spray, season with pepper and roast until golden, 12 to 15 minutes.
3. Transfer half the beans and onion to a bowl with water and mash beans with the back of a fork until smooth. (Add more water to reach desired consistency if necessary.) Spread mixture onto tortillas.
4. Toss remaining beans and onion with zucchini and jalapenos and divide among tortillas. Top each tostada with radishes, cilantro and cheese. Serve with lime wedges if desired.