Dave Cruz learned this technique for preserving mushrooms from a chef in the Basque region of Spain, and it's so tasty and versatile that you'll find many uses for it. The method is simple - mushrooms are steeped in medium-hot aromatic oil for 5 minutes and then seasoned with vinegar and salt. Mushrooms prepared this way are both earthy and acidic, excellent served hot with meats (the Santa Maria-Style Tri-Tip from page 54 of Ad Hoc at Home), or as a base for fish. They are also good cold, as a condiment, as a component of a salad (Mushroom and Leek Salad with Oven-Roasted Tomatoes and Radishes from page 160 of Ad Hoc at Home), or even as the main component, garnished with shaved radishes and similarly preserved tomatoes. Use a variety of mushrooms for textural and taste contrast - oyster, hen-of-the-woods, trumpet, shitake, chanterelle, porcini, and morels. Strain and save the oil when you're finished with the mushrooms to use in vinaigrettes or as a flavored oil.
Just before cooking, rinse the mushrooms to remove any dirt.
Remove the shitake stems and any other stems that are touch and discard, or reserve for another use (such as a vegetable stock).
Trim the ends of the other stems, as well as any bruised areas. Leave small mushrooms whole. Cut larger mushrooms into pieces of into slices; or slice mushrooms with meaty stems, such as porcini or trumpet mushrooms, lengthwise in half, then use the tip of a paring knife to score the inside oft he stem in a crosshatch pattern. The pieces of mushroom will shrink as they cook, but the finished pieces should not be larger than one bite. You should have about 10 cups of trimmed mushrooms.
Combine the olive oil, bay leaves, thyme sprigs, rosemary, and Espelette in a large wide pot, put a deep-frying thermometer in the pot, and heat over medium to medium-high heat until the oil reaches 170°F (it may be necessary to tilt the pot and pool the oil to geta correct reading on the thermometer).
Add the mushrooms to the pot and gently turn them in the oil.
When the oil reaches 170°F again, adjust the heat as necessary to maintain the temperature and cook for 5 minutes, gently turning the mushrooms from time to time. The mushrooms will not initially be submerged in the oil, but they will wilt as they steep.
Remove from the heat, stir in the vinegar and salt and pepper to taste, and let the mushrooms steep in the oil for 45 minutes.
Transfer the mushrooms, oil and herbs to a covered storage container; the mushrooms should be covered by the oil. The mushrooms will keep in the refrigerator. Serve hot or at room temperature.
Notes
From Ad Hoc at Home (page 2260 - photograph on page 237)