Place the butter in a saute pan and melt. Add the thinly sliced onions to the butter. Cook over medium-low heat for 20 minutes, stirring occasionally. Pour a small amount of water into the pan toward the end to help the onions absorb the browned bits from the pan.
Cut the top of the rind off the brie.
Unroll the crescent rounds onto a cookie sheet, not separating each individual strip.
Put the brie round with the rind removed from the top side up. Spoon the onions over the brie. Leave any excess butter in the pan.
Bring the edges up and around to fully incase the brie. Pour the remaining butter over the top of the dough.
Bake on 325 for 12-15 minutes or until the crescent is browned. Let sit for 15 minutes before cutting.