The men and women in the gray flannel suits flock to this sophisticated downtown dining room, with its tufted walls, sleek deco bar, and precise service, to fling back martinis ("Shh . . . don't tell the boss. Hey! I am the boss!) and feast on Marlin Kaplan's sometimes-quirky, always-celebrated, great American cuisine. Whether they're wheeling, dealing, or just feeling like recharging those $200,000-a-year batteries of theirs, they know One Walnut is the spot where business gets done over delights like Lobster Nachos and Pastrami Salmon with goat cheese and caper mayo on rye. That sack lunch isn't looking so good anymore, now is it?