Prepare hot roll mix according to package directions; let dough rise until doubled in bulk.
Peel and thinly slice onions; saute in butter in large skillet, stirring occasionally, for 30 minutes or until onions are limp and golden.
Beat sour cream, eggs and salt until blended; stir in onion mixture.
Punch dough down; let rest for 10 minutes.
Roll dough to 11x15-inch rectangle.
Line greased 1 3 x 9-inch pan carefully with dough; turn up 1-inch edge on all sides.
Pour onion filling into dough-lined pan; sprinkle caraway seed over top.
Bake at 350 degrees for 55 minutes or until crust is browned and custard is set.