Preheat oven to 400.
Peel and slice potatoes, prep onion and garlic, set aside bowl with mixture of flour, salt, and pepper.
Melt butter in a large non-stick sauce pan over medium heat.
Add garlic and onions, cook until tender.
Add flour mixture, toss until coated.
Add milk, then bring to boil.
Cook and stir until slightly thickened, approx 5 minutes.
Place half of potato slices in casserole dish, then cover with half the sauce. Finely grate a dusting of parmesan cheese over this layer.
Add the remaining potato slices, add the remaining sauce, and dust with finely grated parmesan.
Cover dish with foil.
Bake for 70 minutes, then uncover for 10 more minutes.