Quarter potatoes, then place in a large saucepan of cold water set over high heat. Peel and cut celeriac into cubes, then rinse to remove any grit. Add to potatoes along with garlic and a generous pinch of salt. Add more water, if needed, to completely cover vegetables. Cover and bring to a boil. Reduce heat and gently boil until vegetables are very tender, from 25 to 30 minutes. Drain vegetables. Remove garlic to a dish and mash with a fork. Return to potatoes. Mash potatoes and celeriac with a potato masher. Add Dijon and 1/2 teaspoon (2 mL) salt and pepper. Gradually beat in buttermilk. Serve sprinkled with chives or onions.
Nutrition (per serving)
The creaminess of these potatoes comes from the low-fat buttermilk.