Prove NP-Completeness clique + independent set graph

Question

"Prove that it is NP-Complete to determine given input G and k whether G has both a clique of size k and an independent set of size k. Note that this is 1 problem, not 2; the answer is yes if and only if G has both of these subsets."

We were given this problem in my algorithms course and a large group of students could not figure it out. Here is what we have so far...

We know that both the clique and independent set problems are NP-Complete in of themselves. We also know that the verification of this problem, given some "certificate" is in NP.

The problem is somehow performing a reduction on the above problem (which contains both independent sets and cliques) to either a problem consisting entirely of cliques or independent sets (at least that's what we think we need to do). We don't know how to perform this reduction without losing information needed to reduce the reduction back to its original form.