In a large bowl, stir together olive oil, butter, lovage, and rosemary. Add potatoes, onion, salt, and pepper. Toss to coat potatoes. Transfer mixture to a 13x9x2-inch baking pan.
Roast, uncovered, in a 400 degree F oven for 40 to 45 minutes or until the potatoes are tender. Makes 8 to 10 side-dish servings.
If you can't find lovage, use 1 tablespoon snipped celery leaves.