Preheat your oven to 325°F (165°C). Lightly grease an oval glass casserole dish (about 2–3 quarts) with a little of the vegetable oil, using your fingers or a paper towel to coat the bottom and sides.
Peel the potatoes and slice them into thin rounds, about 1/8–1/4 inch thick. The thinner they are, the more they’ll soften and melt into that caramelized, almost unidentifiable mass. Slice the onions thinly from root to tip into half-moons.
Close-up of thinly sliced potatoes and onions on a cutting board
In a large bowl, toss the sliced potatoes and onions with the remaining vegetable oil and the kosher salt until everything is lightly coated and the salt is evenly distributed. This helps the vegetables release moisture and start to break down as they bake.
Layer the potato and onion mixture into the prepared casserole dish, piling it in loosely rather than packing it down. You want some air pockets so the edges can crisp and the top can deeply brown. Scatter the butter pieces evenly over the top and gently tuck a few down into the layers.
Unbaked potato and onion casserole layered in a glass dish
Cover the dish tightly with foil and place it on the middle rack of the oven. Bake covered for 1 1/2 hours, until the potatoes are very tender and the onions are soft and translucent. During this time, the vegetables will release liquid and start to stew in their own juices and the butter.
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