Slow Cooker Texas Chuck Wagon Chili

4 large onions, cut into eighths

4 to 4 1/2 pounds fresh beef brisket (not corned beef), trimmed of fat and cut into 1-inch pieces

1 1/2 tablespoons finely chopped garlic

2 1/2 teaspoons salt

1/2 teaspoon pepper

2 to 4 dried ancho chiles, coarsely chopped

2 tablespoons ground cumin

3 cups Progresso® beef flavored broth (from 32-ounce carton)

2 large yellow, red or green bell peppers, cut into 2 1/2-inch strips

Shredded Cheddar cheese, if desired