In a 9 by 13-inch baking dish, combine the vinegar, soy sauce, oil, brown sugar, paprika, mustard, and garlic powder. Sliced mushrooms are added; whisk to thoroughly coat. Overnight or for eight hours, cover and chill.

Set oven to 250 degrees Fahrenheit. A wire rack should be placed on each of the two sizable baking sheets that have been lined with parchment paper.

Place the mushrooms in a single layer, evenly distributed amongst the prepared racks, being careful not to overlap them. About 1 1/2 hours should be required to roast the mushrooms, switching the pans halfway through. Crisp edges and a chewy feel are ideal. Remove from oven, then allow to fully cool.