This is my dad's favorite drink, and probably mine. There are a few small factors that can drastically alter the character of the drink - type of gin, type and amount of vermouth, type of onions. But basically we're looking at a martini with cocktail onions. My preferences are for dry gin and plenty of vermouth. And I love the Santa Barbara cocktail onions, pickled in vermouth.
2 oz London dry gin (Bombay dry, simple and affordable)
1/2 oz Dolin dry vermouth
(If you're using cocktail onions steeped in vermouth, you should sub in some of the onion-flavored vermouth for the Dolin.)