1. Cut larger mushrooms lengthwise into 2 or 3 slices. Leave smaller mushrooms intact.
2. Heat olive oil in a deep, heavy skillet over medium-high heat. Add mushrooms in a single layer (do several batches if needed) and cook until lightly browned on one side, 3 to 4 minutes.
3. Flip mushrooms, add shallot or onion and cook for 1 to 2 minutes.
4. Lower heat to medium and cook for an additional 2 to 3 minutes. Add a little more olive oil if needed.
5. Add garlic, chopped peppers and thyme and sauté gently for 1 minute. Do not brown the garlic.
6. Season with salt and pepper, then add sherry to deglaze pan. Reduce for 3 to 4 minutes.
7. Remove from heat, add a splash of seasoned rice vinegar and stir until combined.
8. Serve warm or at room temperature. Mushrooms can be refrigerated and reheated.