Stir sourdough starter before measuring out 3/4 C.. Bring the 3/4 C. portion to room temperature. In a large bowl, combine 3/4 C. flour, sugar, undissolved yeast and salt. Heat water and oil until very warm (120-130 degrees). Gradually add water mixture and starter to flour mixture. Beat 2 minutes at medium speed of electric mixer, scraping bowl occasionally. Stir in enough remaining flour to make a soft dough. Knead on a lightly floured surface until smooth and elastic, 8 to 10 minutes. Cover; let rest 10 minutes. Meanwhile, combine tomato paste, water, vegetable oil, oregano, salt and pepper. Stir well. Divide dough in half. Roll each half to a 12-inch circle. Place on greased pizza pans, form a standing rim by pinching the edge of the dough. Prick with fork; let rest 10 minutes. Have oven heating to 450 degrees. Par-bake both crusts at 450 for 7 minutes. Remove from pans; place pizza shells on wire racks. Spread tomato mixture evenly on each pizza crust. Sprinkle with mushrooms, green pepper and cheeses. (If desired, use mixed colors of bell peppers and sliced sweet onions; omit mushrooms. Sauteed vegetables to slightly tenderize before topping pizzas.) Bake on wire rack at 450 degrees for 10 minutes. Serve warm. Yield: Two (12-inch) pizzas, or 8 servings.