My hair is falling out, my nails are brittle, and my eyes have lost their luster. (Or what little luster they had.) Do you think my eating nothing but onion rings for the past four weeks might have something to do with it? Not that I care. I look awful, yes, but I've never been happier. Onion rings are bite and sup to me, especially the ones at Luther's Bar-B-Q (9797 Westheimer, 780-0081), called here a loaf. Have you seen these things? Costing $2.49, they come in a great tangled mass and look like something you'd find in a tackle box. (Do yourself a favor and start with half a loaf. You can always work your way up.) But the taste! Oh, the taste! Lightly battered and gently fried, these onion rings are balm to the soul.