How to Make It
Combine beans and pico de gallo in a small saucepan. Cook over medium, stirring often, until hot, about 5 minutes. Remove from heat, and cover to keep warm.
Whisk together oil, vinegar, oregano, and salt in a large bowl. Add cabbage, radishes, tomatoes, and onion; toss to combine. Let stand 5 minutes.
Spread each tostada shell with 1/3 cup bean mixture; top each with 1 cup cabbage mixture and 1/4 cup queso fresco. Garnish with torn cilantro, if desired.