Mix first five ingredients in a large bowl. Combine beef over mixture and mix well, shape into four oval patties. Grill burgers, covered, over medium heat or broil 4 in. from the heat for 5-7 minutes on each side or until a meat thermometer reads 160 and juices run clear. Meanwhile prepare texas toast according to pkg. directions. Saute onions and red peppers in butter in a large skillet until tender, spoon onion mixture obto toast slices, top each with a burger, avocado slices and cheese spread.