Crush the garlic cloves and cut into ½ inch pieces.
Trim the mushroom stems to an even length. Toss the mushrooms in a bowl with the oil, salt, garlic, thyme sprigs and pepper.
Lay the mushrooms out on a cast iron pan or baking sheet lined with parchment and roast at 425°F for 20-25 minutes or until lightly brown. Rotate the pan 180° after 15 minutes.
Peel the shallot and slice ¼ inch thick. Heat the shallot slices in a dry cast iron skillet until lightly charred (or just saute them quickly). Remove to a bowl and add the vinegar.
Remove the mushrooms from the oven. Discard the thyme sprigs, transfer to a bowl, add the shallots and olive oil and cool.
Double check the seasoning for salt and pepper and adjust until it tastes good to you. Serve warm or room temperature.