In a non-reactive bowl, like a stainless steel salad bowl, combine the salt and the shallot and let sit for 10 minutes. Add the mushrooms, oil, pepper and lemon juice and combine.
Refrigerate until needed.
The relish will keep under refrigeration for 5 days.
Notes
A quick preserve
This is a quick method for preserving beefsteaks for a few days, if you want to preserve them for longer, increase the quantities of oil and switch the lemon juice to a mild vinegar like rice or champagne since they have a controlled PH (more reliable for hermetic storage).
Using other mushrooms
You can make a good mushroom relish out of plenty of different mushrooms. Most wild mushrooms should be cooked before marinating. Many cultivated ones will be fine raw.