Ask HN: How to 'repair' knowledge?

I find myself at a point where I want to solidify the foundations of my knowledge in many areas that are weak (because of classes I had to cram through in university when overloaded, domains I learned "on the job" as fast as possible, etc).

Does anyone have good methods for patching up this knowledge? I find that I am bored by restarting courses of study I know 60-80% of already and even if I slog through, I don't feel that I'm using my time efficiently. For instance, in linear algebra I'm comfortable with the basics and have intuitions about more complex topics (such as computability of various operations) but I would really like a deeper understanding of the entire field. However, I can never find the right "intermediate" places to begin study, or good self-assessment metrics.