Combine ground sirloin, 3 Tbsp. (45mL) Mrs. Dash® Hamburger Grilling Blend, onions, and green peppers in a medium bowl; mix well. Blend together avocado, 1 Tbsp. (15mL) Mrs. Dash® Hamburger Grilling Blend and fat-free mayonnaise in a small bowl. Place burgers under preheated broiler and brown patties on both sides until juices run clear; about 3 minutes per side. Serve hamburger topped with avocado mixture and sliced tomato on bun.