An easy way to gauge understanding is to have your students write about what they've learned. Comics are a lot more fun and easier than a traditional essay.
Have students use their avatar to simply "talk" about what they've learned in a comic. For an easy lesson, they can use the base school content pack backgrounds to do this. Remember it's not necessary to recreate backgrounds from a lesson in astronomy or politics the goal is writing.
Below is an example of a student comic explaining to the class what they've learned about cell division.