1Heat oven to 450°F. In medium bowl, stir Bisquick mix and boiling water until soft dough forms. Gather dough into a ball.
2Place dough on surface lightly dusted with Bisquick mix. Roll dough into 13-inch round. Place on ungreased 12-inch pizza pan; pinch edge to form 1/2-inch rim. Bake 6 to 8 minutes or until light brown.
3Meanwhile, 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 mixture. Bake 1 to 2 minutes or until cheese is melted. Cut into wedges.