Cut stems and center ribs from chard, discarding any tough portions, then cut stems and ribs crosswise into 2-inch pieces. Stack chard leaves and roll up lengthwise into cylinders. Cut cylinders crosswise to make 1-inch-wide strips.

Heat oil and butter in a large heavy pot over medium heat until foam subsides, then cook onions and garlic with 1/2 teaspoon salt, 1/4 teaspoon pepper and pepper flakes, covered, stirring occasionally, until onions begin to soften, about 8 minutes. Add chard stems and ribs, 1/2 teaspoon salt, and 1/4 teaspoon pepper and cook, covered, stirring occasionally, until stems are just tender, about 10 minutes. Add chard leaves in batches, stirring until wilted before adding next batch, and cook, covered, stirring occasionally, until tender, 4 to 6 minutes. Transfer with a slotted spoon to a serving bowl.
Cooks' notes:
· Chard can be washed, dried, and cut 2 days ahead and chilled in sealed bags lined with dampened paper towels.
· Chard can be cooked 4 hours ahead and reheated over low heat on stove or in a microwave oven.