I located a 3 day ASCE Face to Face Seminar nearby (luckily) and talked my company into springing for it. I hear this class is the best way to learn HEC-RAS. ASCE also offers On Demand Webinars on HEC-RAS but they are very specific in scope and not a full background course. I never found a good complete online course besides this
I think the ASCE courses are very overpriced. The best course is at HEC in Davis, Ca. The best way to learn RAS is to use it to solve engineering hydraulics questions. The types of open channel hydraulics and culvert questions like you would find on the PE exam.