A recipe taken from Oded Schwartz's Preserving, but somewhat modified.
500 ml (ca. 17 oz.) white vinegar (I used 5% apple vinegar)
250 ml water
3-4 garlic cloves, chopped
1 tsp. unground black pepper
2 tsp. salt
4-6 thyme twigs (I didn't have any so I used fresh rosemary from our herb garden)
1 kg mixed mushrooms (I used 75% red-caps, 25% porcini (Boletus edulis))
1-2 strips lemon rind
1 bay leaf (I didn't have any leaves left so I used powder)
olive oil (I used a mixture of ca. 50% extra virgin olive oil , 50% pumpkin seed oil)
1. Pour vinegar and water into stainless steel pot. Add garlic, pepper, salt, half of thyme twigs. Bring to boil, reduce heat and cook for 30 minutes.
2. Add mushrooms and cook over slow heat for 10 minutes or until tender. Remove mushrooms, drain.
3. Put mushrooms, lemon rind, remaining thyme twigs, and bay leaf into a hot, sterile jar.
4. Heat olive oil to 75 degrees C and pour over the mushrooms so that they are covered completely. Probe with a wooden stick to eliminate any air bubbles. Close jar tightly. Consume after no less than 2 weeks.
So after a fortnight or so I may open one to tell you how it's worked out...