At university, I had to write an essay (2 thousands words or so) every week or two on the subject we were currently studying. Then I had to talk about them for an hour or so with someone far better informed on the topic than me. I retained far, far more about subjects by doing this than I did about subjects which I just read about or went to lectures on: even though a lot of time was used in apparently less optimal ways (skimming for things to quote, writing the actual essays to be elegant as well as make the relevant arguments etc.)

As a caveat to this, I should say that the subject was often very subjective (I wasn't embedding fundamental complex truths, more taking sides on debates: the most rigorous it got was analytic philosophy and science-being-talked-about-by-a-humanities-student), and that I really enjoy those sort of arguments, so I might be predisposed towards them.

For me, this way of learning things is a bit like realising that I can be incredibly creative (in the sense of making up arguments and crystallising my thoughts) in a test situation: I know it works, but I find it very difficult to force myself into the artificial situation of having to do it. If I need to in the future for some reason, I think I'd need to find a buddy or something to provide pre-commitment.