Spinach, Feta, and Potato Gratin

butter (, room temperature, for baking dish)

medium-sized red potatoes (, washed, peeled, and sliced 1/8-inch thick, divided)

bags ((10-ounces) fresh baby spinach OR 3 cans (13.5-ounces) spinach, washed and drained, dry with paper towel SEE NOTES FOR DIRECTIONS ON USING FRESH SPINACH)

crumbled feta cheese

heavy cream

eggs

salt and pepper