Select very fresh unopened mushrooms. Wash and trim stems, leaving 1/4 inch attached.
Add bottled lemon juice and enough water to cover. Bring to a boil, simmer 5 minutes, and drain.
Mix oil, vinegar, oregano, basil, and salt. Stir in onions and pimento and heat to boiling.
Place 1/4 garlic clove and 2 to 3 peppercorns in each half-pint jar.
Fill jars with mushrooms and hot, well-mixed oil/vinegar solution, leaving 1/2-inch headspace.
Process half-pints 20 minutes at 0–1,000 ft, 25 minutes at 1,001–3,000 ft, 30 minutes at 3,001–6,000 ft, or 35 minutes above 6,000 ft.