Melt butter in a medium-sized pot on the stove top over medium-high heat.
Add the onion and garlic and heat, stirring until tender, for 3-4 minutes.
Pour 1 ½ cans of evaporated milk into the pot. Whisk in bay leaves, salt, thyme, pepper, dry mustard and rosemary. Bring to a boil, then reduce heat and simmer for 3-5 minutes.
Butter or grease an 11x7 casserole dish. Arrange the potato slices in the bottom of the pan.
Take the bay leaves out of the milk mixture. Stir in Parmesan cheese until combined.
Pour the milk mixture over the potatoes. Cover the dish with aluminum foil, pulling back one corner for steam to escape.
Bake for 40 minutes before taking off aluminum foil and cooking for an additional 10 minutes, or until fork tender.