1. In a large skillet, heat olive oil over medium-high heat. Add red onion, halved and thinly sliced; cook, stirring often, until browned, 3 to 5 minutes.

2. Add button mushrooms, trimmed and thinly sliced, dried thyme, coarse salt, and ground pepper. Reduce heat to medium; cover, and cook until mushrooms are tender, about 10 minutes.

3. Remove lid; cook until liquid has evaporated and mushrooms are browned, about 2 minutes more. Serve warm, or let cool before refrigerating, covered. Reheat gently on the stove (or in the microwave).