Place potatoes and celeriac in a Dutch oven; cover with water. Bring to a boil. Stir in 1/4 teaspoon salt; cook 20 minutes. Add Swiss chard; cook 10 minutes or until tender. Drain well; place potato mixture in a large bowl. Partially mash potatoes with a potato masher; stir in 1/4 teaspoon salt, onions, butter, and pepper.