*If you have a really large cast-iron skillet (like 12-inches) you can use 12 ounce mushrooms. More surface area means all of the mushrooms can sear in an even layer. If you're using a 10-inch skillet or smaller, use 8 ounces of mushrooms. Or, sear the mushrooms in oil in batches before adding them all back to the skillet with the butter.
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