ISO of home made onion rings
Jack's Prime in the southern part of San Mateo makes their own rings, and for the month of December has a special "black and tan" onion ring made with Guinness in the batter. I didn't especially care for them but if you like Guinness, you probably would.
Rave Burger in downtown San Mateo also makes their own rings and onion strings; I have not had the rings, but the strings are wonderful. Very fine sweet potato fries at both of these establishments too.
144 E 3rd Ave, San Mateo, CA 94401
3723 S El Camino Real, San Mateo, CA 94403
I will issue a fuller report later, but I ran into the Southern Sandwich Company truck at Marina Blvd. in Brisbane, and while their beer battered rings may not have been as good as Woodman's in Ipswich Mass. (my personal gold standard) they were the best I've ever had from a truck, and cost-get this- one dollar for an order of about 6. Not a restaurant, but worth seeking out.