Hi blindfoldjump, usually the quickest way for something like that is to form the notched out area by a boolean operation rather than filling things in "patch by patch". That way the cutting object leaves a portion of itself behind and then you don't have holes like that to fill in.

But to fill it in like you've got there currently one quick way would be to draw in a line like this:



Now you 2 four-sided areas each of which can be filled in by selecting 4 boundary curves and running Construct > Network, or if you have a planar area use Construct > Planar.

- Michael