Expression “一...就...” - what if there is a sub-sentence?
Would 我一看这很容易的课就懂了。 be correct?

The sentence is grammatical, However, 这课我一看就懂 already indicated you think it is easy, adding &quot;很容易的&quot; make the sentence unnecessarily wordy.
You can add relative clauses in any part of the sentence as long as it makes sense:
这油画我一看就知是假的
这(标价三千万的)油画，我一看就知是假的
这油画(用的是二十世纪才有的油彩)，一看就知是假的
这油画，(身为画家的)我一看就知是假的 -