If you had a plasma cutter, you would not have asked the question. So
I am assuming that you don't. The Drill / Mill should work well once
you figure out how to clamp the part. Lowering the head so the column
is short will help on the rigidity.

The best way would be to use a hole saw or a 13/16th drill. Drilling
is one of the most efficient ways to remove metal. The forces are
balanced and both lips are continuously cutting. Use coolant and
fairly low speed. Be sure to keep cutting at a feed fast enough that
you are under the metal that has been work hardened on the previous
revolution. Not feeding enough is a mistake. If you have to sharpen
your drill, give it a little extra clearance.

Carpenter Tech has a great booklet on machining Stainless Steel.

Dan