Champ -- a Mashed Potato Dish (Ireland)

Name: Champ -- a Mashed Potato Dish (Ireland)
Prep time: 30 min
Cook time: 30 min
Total time: 30 min
Yield: 4
Cuisine: Global
Ingredients: 355 milliliters buttermilk (actually almost 1 1/2 cups); 4 green onions, trimmed; 0 teaspoons ground pepper, to taste; 1 medium onion, sliced thin; 8 mediums potatoes, peeled, sliced; 0 teaspoons salt, to taste; 2 Tbsps unsalted butter