Brown hamburger along with half the package of onions in a large skillet. Drain well. Place back in skillet and add taco seasoning according to package directions. Allow to cook until most of the fluid is cooked off. Re-drain hamburger.
In mixing bowl add sour cream ,mayo, the rest of the package of onions and 1 cup of cheese. Mix until well blended.
In a 9x13 UNGREASED baking dish spread hamburger evenly over bottom. Spread sour cream mixture evenly over hamburger layer.Spread both cans of Rotel (make sure these have been very well drained) over sour cream layer. finally cover with with remaining 2 cups of shredded cheese.
Mix together milk,eggs and Bisquick removing as many lumps as possible.This will be very runny. Pour evenly over top of casserole.
Bake in preheated 400 degree oven for 45 minute to an hour or until Bisquick is completely done. Allow to set for 10 minutes before serving.