In medium bowl, stir Bisquick mix and boiling water until soft dough forms. Gather dough into a ball. Place dough on surface lightly dusted with Bisquick mix. Roll dough into 13" round. Place on ungreased 12" pizza pan; pinch edge to form 1/2" rim. Bake 6-8 minutes or until light brown.
Meanwhile, in medium bowl, mix tuna, dressing and onions.
Spread tuna mixture over crust. Arrange tomato and cheese slices alternately in a pinwheel pattern on tuna. Bake 1-2 minutes or until cheese is melted.
Cut into wedges and serve.