Talk about umami! Whenever mushrooms, onions, and garlic are cooking, that’s when everybody in the house is salivating for dinner, because the scent is so intoxicating. This particular dish is so crave-worthy, you might want to make a double batch!
Skillet-roast the garlic cloves: Heat a large skillet over medium heat and add the unpeeled garlic cloves. Cook, stirring occasionally, so that the skin gets charred on all sides, 15 to 20 minutes. Remove from the heat and wrap the garlic cloves in a clean kitchen towel for a few minutes to cool and soften. Peel off the skins and set aside.
Heat the same skillet over medium-high heat and add the mushrooms to the dry skillet. Cook, stirring occasionally, until the mushrooms have released their natural juices and begin to soften and brown, about 10 minutes. Remove the mushrooms from the skillet and set aside.
Melt the butter in the same skillet, then add the thawed pearl onions and the peeled skillet-roasted garlic cloves. Sprinkle the sugar and 1 teaspoon of salt over the top of the onions and garlic, then toss to coat completely in the butter. Cook, stirring occasionally, over medium-high heat until the onions begin to caramelize and turn golden, 5 to 7 minutes, then add the reserved mushrooms back into the pan, tossing with the onions and garlic.
Whisk the vinegar into the vegetable stock, then add that to the skillet, along with the thyme sprigs. Simmer over medium-high heat until the liquid begins to thicken, 2 to 3 minutes, then stir in the cream, nutmeg, and Parmesan to coat and bring to a simmer so that the sauce warms through completely, 2 minutes. Add salt and pepper to taste before serving.