Preheat your oven to 190 C. Grease a large casserole dish with butter.
Slice the potatoes in half lengthwise, then slice each half into 1/4 inch slices.
Cook the spinach in a saucepan according to package instructions. Drain and squeeze out excess water. If using leaf spinach, lightly chop the spinach after draining and squeezing it dry.
In a large bowl, combine potato slices and olive oil and toss to coat. Add garlic, oregano, basil, and salt and toss to coat.
In prepared casserole dish, coat bottom with 1/3 of the pasta sauce, 1/3 of the potatoes, 1/3 of the spinach, 1/3 of the Parmesan, and 1/3 of the Mozzarella. Repeat layers 2 more times.
Bake uncovered 45 minutes or until potatoes are tender.
Serve, perfect sides would be a green salad and garlic toast.