In a large saucepan, cover potatoes with cold water and bring to a boil over medium-high heat. Reduce heat and cook for 20 to 30 minutes, or until tender. Meanwhile, melt 2 tablespoons (30 mL) of the butter in a large skillet over medium-low heat. Add chopped onion and cook, stirring occasionally, until softened, 8 to 10 minutes. Turn heat to medium and cook, stirring occasionally, for about 5 minutes more, or until golden.

When cooked, drain potatoes and reserve ½ cup (125 mL) of the cooking water. Return potatoes to the pot, along with the reserved water and remaining butter. Season to taste with salt and pepper.

Mash potatoes until well blended. Stir in onions and parsley.