Step 1

To prepare crema: Combine sour cream, lime zest, lime juice and salt in a small bowl. Set aside.

Step 2

To prepare tostadas: Position a rack in upper third of oven; preheat to 400 degrees F.

Step 3

Brush both sides of tortillas with 1 tablespoon oil and arrange on a baking sheet. (It's OK if they overlap a bit; they will shrink as they cook.) Bake, turning once halfway, until browned and crisp, about 10 minutes. Transfer to a wire rack and let cool.

Step 4

Meanwhile, heat 2 teaspoons oil in a large skillet over medium heat. Add 1 garlic clove and cook, stirring occasionally, until fragrant, about 30 seconds. Add cumin, 1/2 teaspoon salt and chile powder; cook, stirring, for 30 seconds more. Add beans and cook, stirring occasionally, until heated through, about 4 minutes. Transfer the beans to a food processor and add 1/4 cup water. Pulse until smooth, adding more water, 1 tablespoon at a time, if needed.

Step 5

Preheat broiler to high.

Step 6

Toss bell peppers, onion, zucchini, corn, ground pepper, the remaining 3 garlic cloves, 1 tablespoon oil and 1/2 teaspoon salt in a large bowl. Spread on a large rimmed baking sheet. Broil, stirring occasionally, until lightly charred, 8 to 12 minutes.

Step 7

Top the tostadas with some of the beans, charred vegetables, cabbage, cilantro, cheese and the reserved crema.