1) A flush-trim bit and a dedicated rabetting bit or 2) A rabetting bit set that has multiple bearings so it can vary the depth of cut from 0 (flush) to 1/2".
It's probably a little cheaper, and more versatile, to get the bit set. I've seen them by Freud and Woodcraft for about $45.
Are the sets as accurate as the dedicated bits? Is it a huge pain to have to swap bearings to change the depth of cut, or is it no harder than adjusting a fence? Will it flush-cut as accurately as a dedicated flush-trim bit?