Preheat oven to 425 degrees.
Slice the potatoes and onion into 1/4 inch slices. I recommend using a Mandoline, if you have one.
Line the slices into a greased 9X13 inch pan, alternating the sweet potato then white or red potato slice and every few potatoes add in an onion slice.
Mix the melted butter, olive oil and FGF Herbal Vinaigrette seasoning together and drizzle over the potato and onion mixture.
Bake for about 1 hour or until the potatoes are crispy on edges and a fork can go through them.