Place potatoes in a large saucepan and cover with water. Bring to a boil. Reduce heat; cover and cook for 15-20 minutes or until tender., Meanwhile, in another large saucepan, bring 1/2 in. of water to a boil. Add spinach; cover and boil for 3-5 minutes or until wilted. Drain and coarsely chop; keep warm., Drain potatoes and return to the saucepan. Add milk, butter, salt and pepper; mash until smooth. Fold in cheese and spinach.