Butter the sides of a 1 1/2 to 2 quart casserole dish.

Place the potatoes in a sauce pan and cover with water. Bring to a boil, lower heat and simmer until fork tender (but not soft), about 20 to 30 minutes. Drain and cool.

If you are using fresh spinach, roughly chop, and steam for about 5 minutes, or until tender. Drain and run under cold water until cool.

Preheat oven to 375°F.

Melt the butter in a small saucepan over medium heat and cook the onions until tender, about 5 minutes. Add flour and cook, stirring constantly, about 5 minutes. Add milk in three additions, and cook until bubbly and thickened, about 15 minutes.

Peel potatoes.

Slice potatoes about 1/4-inch thick and layer about half of each type of potato in the casserole dish. Top with all the spinach, then about half of the cream sauce. Layer with the remaining potatoes and top with remaining cream sauce.

Cover and bake for 30-40 minutes, or until bubbly in the center. Uncover and bake 15 minutes longer, or until top is browned.