Caramelized onions can top almost anything.
Add the onions and cook, covered, for 15 minutes, stirring occasionally. Uncover the skillet and cook for 5 minutes more or until golden. While the onions cook, set a large pot of lightly salted water to boil on the stovetop. Raise the heat under the onions to medium-high, add the mushrooms, and cook, stirring frequently, until most of their liquid has evaporated. Stir in the garlic, thyme, salt, and pepper.