Potato Spinach Bake

1 10 oz. box spinach, thawed
2 lbs yellow potatoes
3 tablespoons butter
1/4 cup onion, finely chopped
2 large garlic cloves, minced
3 tablespoons all-purpose flour
2 1/2 cups milk
1 teaspoon (salted) chicken or vegetable broth concentrate
1 teaspoon salt
white pepper
Grated Parmesan cheese for sprinkling

1. Slice the potatoes and boil in salted water until tender, about 12 minutes. Drain.
2. Squeeze the extra liquid out of the thawed spinach.
3. In the same pot you cooked the potatoes in, heat the butter over medium-low heat and cook the onion and garlic a few minutes until softened but not browned. Stir in the flour. Slowly add the milk while whisking. Bring to a low boil, whisking constantly. Stir in the broth concentrate, salt, and white pepper to taste. Cook until thickened.
4. Heat oven to 350°F. Spread half the potatoes in a greased 8 x 8 baking pan. Evenly spread the spinach over the potatoes. Pour half the milk mixture over the spinach. Layer the remaining potatoes over the spinach and pour the remaining sauce over the potatoes to coat all the potatoes. Sprinkle with a little Parmesan cheese if desired.
5. Bake for 30 – 45 minutes until bubbly and lightly browned on top. To keep any drips from messing up the oven, you can place the pan on a baking sheet.