Very Veggie Burgers - Meatless Monday

Ingredients

• 2 cups mushrooms, sliced
• 1 tablespoon unsalted butter
• 1 1/2 cup sweet onions, sliced, divided
• 1 can (16 oz.) black beans, drained, rinsed
• 1 cup red bell pepper, seeded
• 2 cloves garlic, peeled
• 1 large egg
• 1/8 teaspoon salt
• 2 tablespoons sweet chili sauce
• 3/4 cups whole wheat bread crumbs
• 4 whole wheat buns