Reuben En Croute

1/2 C chopped onion
1/4 lb. shredded swiss cheese
1C sauerkraut, well drained
1/4 C butter, melted
8 sheets phyllo dough
3/4 C sour cream
1 T sweet pickle relish
1/2 C dijon mustard

In a saucepan, over medium heat, cook onion in 1 T butter until tender. Stir in sauerkraut, 1/4 mustard. Melt remaining mustard. Stack phyllo dough sheets, brushing butter between each layer. Top with layers of corned beef, cheese, sauerkraut mixture to within 1" of edges. Roll up from long edge, jelly roll fashion, tucking in ends and brushing seams with butter. place roll seam side down on a greased baking sheet. Curt slits on top of roll in 12 even sections, about 1/2" deep. bake at 375 degrees for 30-35 minutes or until golden brown.
For the sauce:blend remaining mustard, sour cream and pickle relish. Slice roll and serve with sauce.