Preheat oven to 425°F.
Combine 1 cup flour, undissolved yeast, sugar and salt in a large bowl. Heat milk and butter until very warm (120° to 130ºF). Add to flour and mix until well blended, about 1 minute. Gradually add enough remaining flour to make a soft dough. Dough should form a ball and will be slightly sticky. Knead** on a floured surface, adding additional flour if necessary, until smooth and elastic, about 4 minutes. (If using RapidRise Yeast, let dough rest at this point for 10 minutes.)
Pat dough with floured hands to fill greased pizza pan or baking sheet. OR roll dough on a floured counter to 12-inch circle; place in greased pizza pan or baking sheet. Form a rim by pinching the edge of the dough. Cover with a towel and let rest 10 minutes.
Toss raspberries and blueberries with sugar and corn starch and let rest while preparing streusel topping.
Mix flour, brown sugar, cinnamon and nutmeg in a bowl. Cut in butter with pastry blender or two forks until crumbly. Stir in oats. Spread fruit over pizza crust. Sprinkle streusel over fruit.
Bake on lowest oven rack for 12 to 15 minutes, until topping and crust are browned. Remove from oven and let cool on wire rack for 5 minutes.
Combine powdered sugar, vanilla and enough milk to make a smooth consistency. Drizzle over pizza. Serve immediately.
**To knead the dough, add just enough flour to the dough and your hands to keep the dough from sticking. Flatten dough and fold it toward you.Using the heels of your hands, push the dough away with a rolling motion. Rotate dough a quarter turn and repeat the "fold, push and turn" steps. Keep kneading dough until it is smooth and elastic. Use a little more flour if dough becomes too sticky, always working the flour into the ball of dough.