Yep, thaw it gently.
That small a flat I'd season, wrap with bacon, and cook gently at 225-250. Get some good color on it (about 2 hours) and then I'd definately wrap it in foil and continue cooking until tender.
The general rule with brisket is 1-1.5 hours per pound, but that's for a packer with lots of fat and connective tissue.
I've had good success with flats in the past, but I haven't done one in a long time. Watch it like a hawk or it'll get dried out.