In a skillet over med-low heat, add oil. Add onion and cook till transparent, stirring occasionally, ~5 minutes.
Increase heat to medium and add mushrooms, salt, and pepper. Cover and cook ~5 minutes, allowing the liquids to release from the mushrooms.
Uncover, increase heat to high, cook and stir frequently until liquid evaporates and mushrooms brown (~5 minutes).
Add thyme and stock, cook until skillet nearly dry, ~1-2 minutes.