The code you have is incorrect. GetAllStocks is a method that the client can invoke on the server (e.g. it's something the server exposes). You can do:
var stocks = await stockTickerMini.Invoke<IEnumerable<Stock>>("GetAllStocks");
If you want to get called back when stocks are being updated then you need to look at what the server side is calling back on the clients object:
Which is detailed right here:
http://www.asp.net/signalr/overview/hubs-api/hubs-api-guide-net-client#establishconnection