Interbay: 1613 W Dravus St, Seattle, WA, T: (206) 284-6363
And talking about sides, not taking anything away from their burgers, but oddly enough, Red Mills Burgers stands out in its... onion rings! I am not even a fan of onion rings, but their Babe's Onion Rings ($2.59) are to die for! Do add a side of Tub O' Tartar Sauce ($0.23). If you have any space left, get a side of french fries ($1.89); as the onion rings, they are fried in 100% peanut oil. The fries are not special, but they are very decent. Finally, do not leave without a shake; they have many different tastes at $3.19 (I love their banana shakes!).
UPDATE: I can also vouch for the Red Onion Jam Burger, with baked red onions, and the BLT (on whole wheat toast and with thick peppered bacon). Both are YUM!
The onion rings are, without a shadow of a doubt, the best I ever ate. Absolutely amazing! The fries are good, but no competition for the onion rings. The burgers are among the best I ate at a pig out spot, though I wasn't that impressed by the verde or bbq burgers. The milkshakes were really creamy and tasty.
If you are in Seattle, you MUST go to Red Mill Burgers! And once you are there, and have waited your turn (there is ALWAYS a serious wait!), do get whatever burger you want and do NOT leave without the onion rings and tartar sauce!