1/2 teaspoon salt
1/2 teaspoon freshly ground black pepper
4 whole-grain hamburger buns
Heat the oil in a small nonstick skillet over medium heat. Add onions and saut until just softened, about 3 minutes. Add the garlic, cumin and chipotle and saut until fragrant, about 1 minute longer. Transfer to a dish to cool.
Meanwhile, in a medium bowl, combine the beans, bread and tomato paste. Use a potato masher or fork to mash them into a rough paste. Add the ground beef, cilantro, salt, pepper and the reserved onion mixture. Mix thoroughly, then shape into four 3/4-inch-thick patties.
Grill the burgers on a well-oiled rack, or broil them until they register 160 degrees on an instant-read thermometer and there is no trace of pink inside, about 5 minutes per side. Meanwhile, toast the buns on the edge of the grill or in a toaster.
Nutrition information per serving: 388 calories; 15 g fat (5 g saturated); 52 mg cholesterol; 39 g carbohydrates; 23 g protein; 4 g fiber; 681 mg sodium.