Chard with Leeks

1 bunch rainbow, red or green swiss chard
1 ½ Tbsp. butter
1 Tbsp. olive oil
1 bunch leeks, halved lengthwise and cut crosswise into
1/4" thick slices (white & pale green parts)
2/3 tsp. fine sea salt
Pepper

Cut and reserve stems from chard (if leaves are large, also cut out coarse portions of rib), then cut stems and ribs crosswise into 1 inch slices. Stack chard leaves & roll into cylinders. Cut cylinders crosswise to make 1 inch thick strips of leaves. Heat butter and oil in skillet over moderately high heat until foam subsides, then saute chard stems & leeks with sea salt and pepper to taste, stirring often, until slightly soft, about 5 minutes. Add leaves and continue to saute, stirring often, until wilted. Serves four.