Preheat oven to 425 degrees F. Lightly grease jelly roll pan, 15 1/2 x 10 1/2 x 1- inch, or a cookie sheet.
Mix baking mix and water until soft dough forms; beat vigorously 20 strokes. Gently smooth dough into ball on floured board. Knead 20 times. Using a rolling pin, roll into a rectangle measuring 15 1/2 x 10 1/2- inches.
Place dough on and up sides of the pan with floured hands. Or place on cookie sheet; pinch edges of rectangle, forming 3/4- inch rim.
Cook and stir lean ground beef, onion, salt and garlic until beef is brown; drain grease.
Mix tomato sauce and Italian seasoning; spread over dough. Scatter cheese over tomato sauce.
Spoon beef mixture over cheese and top with mushrooms and peppers.
Bake until crust is golden brown; about 20 minutes. Slice into 3 x 3- inch slices and serve.