Preheat the oven to 350F/180C

Run the garlic clove around the baking dish, then either toss it, or do as I do and leave it in the dish.

Butter the baking dish. I use about a tablespoon of butter.

The potatoes may be peeled, sliced, and parboiled until just soft–not mushy–or microwaved for six minutes. If you microwave the potatoes, let them cool until comfortable to handle, then peel and slice into rounds.

Place a layer of onions in the baking dish.

Place a layer of potatoes in the baking dish.

Add a layer of spinach or whatever greens you are using. I had enough spinach for a single layer.

Add another layer of potato.

Add ham, if using.

Keep layering the ingredients, finishing with potatoes on top. The breadcrumbs will be added later.

Salt and pepper the gratin.

Place baking dish on baking sheet. Add buttermilk to dish. The liquid should rise just below the top layer of potatoes; you don’t want the liquid sloshing everywhere.

If you are short buttermilk, top up with regular milk or broth. If you have neither, even water will work.

Carefully slide the gratin in the oven and bake for an hour, checking to ensure it isn’t boiling over (it shouldn’t at this temperature) or burning. If the potatoes are burning, cover them with foil.

After an hour, test for doneness by poking a potato with a fork or other tester. If the gratin is close to done, add the breadcrumbs. If it needs more than ten minutes cooking, wait. Add the crumbs at the very end of baking, perhaps five minutes or so.

The gratin is done when a fork or tester slides into a potato without resistance.

Buttermilk Potato Gratin may be prepared up to three days ahead of serving. Bake it, allow it to cool, then cover tightly with tinfoil or plastic wrap and refrigerate. On the day you plan to serve the gratin, bring it to room temperature, and either reheat it in a low oven, or portion it on plates and microwave it.

Leftover Buttermilk Potato Gratin keeps, refrigerated in covered container, up to four days. I keep leftovers in the baking dish, covered with foil.

Leftovers may be frozen, but the texture will suffer.

Notes:

The cooked gratin may appear separated. There is nothing wrong with it, and it will still taste delicious.

As I live with an onion hater, I used only half, placing it on half the gratin.

I used one and one-half bottles of buttermilk. They were 16 ounces/474ml each.

Buttermilk potato gratin may be prepared with any type of dairy–at which point it loses it’s name, but that’s okay. I have not tried nondairy milks, so cannot speak to how they’ll work, but by losing the ham and using them, the dish becomes entirely vegan.

Feel free to substitute/add other root vegetables like turnips, rutabagas, carrots, or parsnips. They will need trimming, peeling, and either parboiling or microwaving before baking.

Use shallot or other alliums in place of the onion.