Chop onions using only 1" of green tops. Saute in 1 T of butter until glazed. Transfer to crock pot. Coat chicken breasts in seasoned flour. Saute in butter until brown on both sides. Transfer to crock pot. Drain pineapple. Pour juice over chicken. Cover and cook on low 3-4 hours (high 1 1/2 2 hours) or until breasts are tender. Saute pineapple slices in butter until golden. Place on heated plater. Top each with a chicken breast. Slice avocado into 8 lengthwise strips. Place 2 strips on each chicken breast. Serve pan drippings over rice.