Melt the butter in a 12-inch skillet over medium heat. Add the onion and thyme and cook, stirring occasionally, until the onion is dark brown, 15 to 20 minutes—it should be darker than golden, but not black. Stir in the cremini and shiitake, 1/2 tsp. salt, and 1/2 tsp. pepper; cook, stirring occasionally, until the mushrooms are soft and golden and the liquid they release has evaporated, 8 to 10 minutes. Add the sherry and cook, scraping up the browned bits from the pan, until almost evaporated, about 1 minute. Let the mixture cool to room temperature.
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