Cut stems from chard (if leaves are large, cut out coarse portions of rib), then cut stems crosswise into 1/4-inch-thick slices.
Stack chard leaves and roll into cylinders. Cut cylinders crosswise to make 1-inch-thick strips of leaves.
Heat butter and oil in a 12-inch heavy skillet over moderately high heat until foam subsides, then sauté chard stems and leeks with sea salt and pepper to taste, stirring occasionally, until slightly soft, 3 to 5 minutes.
Add chard leaves and continue to sauté, stirring frequently, until wilted. (If greens begin to brown before they wilt, sprinkle with a few drops of water.).