Heat the oil and butter in a large pan over medium-high heat. Add the swiss chard stems, chopped onion, garlic, salt, and pepper. Cook, stirring frequently, until the onions and Swiss chard stems have softened and the onions start to slightly brown, about 6-7 minutes.
1 Tablespoon extra-virgin olive oil, 1 Tablespoon salted butter, 1 medium onion, 2 cloves garlic, 1/2 teaspoon salt, 1/4 teaspoon freshly ground black pepper