You don’t have to pay $5 a cup for good pickled mushrooms, and in time you can come up with specific recipes to use for any given occasion. In the meantime, here’s a good general-purpose recipe from an old friend of mine who ran a catering business in Oxford. He looked like a red-headed Rick Astley and used a cigarette holder.
Sauté in olive oil 1 pound of mushrooms, sliced or cut in quarters, with 1 large red pepper, cut into strips, one a large red onion, sliced, and 3 cloves garlic, minced. Cool, add balsamic vinegar, thyme, and fresh ground pepper. Salt to taste. Refrigerate before serving.