Bake TATER TOTS as directed on package.
Meanwhile, heat oil in large nonstick skillet over medium heat. Add onions; cook 15 to 20 min. or until golden brown, stirring occasionally.
Spoon TATER TOTS into 11x7-inch baking dish; top with onions and Singles.
Bake 3 min. or until Singles are melted. Meanwhile, mix mayo, ketchup and relish until blended.
Drizzle mayo mixture over Totchos before serving.