Southwestern Bison Burger with Chipotle Mayonnaise

Ingredients

• 1 pound ground bison (buffalo)
• 1 small bunch scallions, finely chopped, mostly white with some green
• 1/2 teaspoon sea salt
• 1/2 teaspoon ground black pepper
• 1 teaspoon ground cumin
• 1/2 teaspoon hot sauce (such as Tabasco), or to taste
• 2 tablespoons chopped fresh cilantro
• 1 small red bell pepper, cored and cut into 1-inch wide strips
• 1 small yellow bell pepper, cored and cut into 1-inch wide strips
• 1/3 cup low-fat canola mayonnaise
• 1 medium canned chipotle pepper in adobo, seeded and minced
• 4 whole-wheat burger buns
• 1/2 small very ripe avocado, mashed
• 4 small lettuce leaves
• 4 thin red onion slices (optional)