Heat the vegetable oil in a large skillet over medium-high heat. Add the ground beef, half of the minced onion, 1 teaspoon salt and the chipotle chile powder; cook, stirring and breaking up the meat with a wooden spoon, until the meat is browned and the onion is tender, about 7 minutes.
Add the tomatoes, beans and zucchini and cook, stirring occasionally, until the zucchini is just tender, about 6 minutes.
Meanwhile, toss the coleslaw mix, lime juice, and salt and pepper to taste in a bowl.
Divide the tostadas among plates and spoon the beef mixture on top. Top with the cheese, slaw and the remaining minced onion. Serve with lime wedges.