Preheat oven to 350°F.

Wash mushrooms and pat dry. Trim the bottom of the mushroom stems, if needed.

In a large bowl, toss mushrooms with 1 tablespoon of oil, ½ teaspoon salt, and minced garlic.

Spread mushrooms on a sheet pan and roast for about 10 minutes, until softened. Set aside to cool.

Meanwhile, in a saucepan over medium-high heat, stir together remaining teaspoon of salt, water, vinegar, and sugar. Heat and stir until sugar and salt are fully dissolved. Remove the pan from the heat.

Distribute sliced garlic, peppercorns, and sprigs of thyme between several sterilized canning jars. Fill with the roasted mushrooms.

Pour the brine over the ingredients in each jar, filling to cover and allowing enough room for placing the lids on the jars.

Store in the refrigerator until ready to use.