Combine 1 cup flour, undissolved yeast, sugar and salt in a large bowl. Add very warm beer; 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.
Pat dough with floured hands to fill greased pizza pan or baking sheet. OR roll dough on a floured counter to a 12-inch circle; place in greased pizza pan or baking sheet. Form a rim by pinching the edge of the dough.
Brush dough with Dijon mustard and then pizza sauce. Top with 1 cup shredded mozzarella cheese.
Slice bratwurst links into 1/4-inch slices. Layer on top of mozzarella cheese. Sprinkle with onions and sauerkraut, if desired. Top with remaining mozzarella and cheddar cheese.
Bake on lowest rack for 14 to 17 minutes, until cheese is bubbly and crust is lightly browned.
*If you don't have a thermometer, water should feel very warm to the touch.
**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.
*** To precook your own beer brats, place bratwurst in saucepan filled with 12 ounces beer. Heat on medium-high, cover and cook 15 minutes until an internal temperature of 180°F is reached. Remove brats from saucepan and cool 5 minutes.