1Heat oven to 450º. Stir Bisquick, boiling water and onions in medium bowl until soft dough forms; beat vigorously 20 strokes.
2Press dough in ungreased 12-inch pizza pan, using fingers dusted with Bisquick; pinch edge to form 1/2-inch rim. Bake about 10 minutes or until light brown. Cool 10 minutes.
3Stir cream cheese, sour cream, dill weed, garlic powder and tuna until blended; spread evenly over crust. Refrigerate 1 to 2 hours or until chilled. Just before serving, top with vegetables and cheese. Cut into wedges.