3 pounds (about 5 large) potatoes, peeled, quartered
1/4 cup (1/2 stick) butter
1 1/4 cups whole milk
5 ounces soft fresh goat cheese, crumbled
1 cup (packed) chopped arugula leaves (about 2 large bunches)
Cook potatoes in large pot of boiling salted water until tender, about 30 minutes. Drain. Return potatoes to pot. Stir over low heat until excess moisture evaporates. Add butter; mash potatoes until smooth. Bring milk to simmer in saucepan. Remove from heat. Add goat cheese; whisk until melted. Add milk mixture to potatoes; whisk until smooth. Stir in arugula. Season with salt and pepper.