Sweet Potato Tostadas

Name: Sweet Potato Tostadas
Ingredients: 3 sweet potatoes (about 1 1/4 pounds) peeled and cut into 1-inch chunks; Kosher salt and freshly ground pepper; 1/4 small head red cabbage, shredded (about 1 1/2 cups); 3 tablespoons fresh lime juice (from about 2 limes); 2 cups chopped pineapple; 1/4 cup fresh cilantro, chopped; 2 cloves garlic; 1 canned chipotle chile pepper in adobo, plus 1 to 2 teaspoons sauce from the can; 1/2 teaspoon ground cumin; 1 15-ounce can refried black beans (vegetarian, if desired); 8 tostada shells; 1/4 cup Mexican crema or sour cream