What would be the best way to cook a london broil so it is juicy AND tender?

Where I live, top round sold in a thick slab is called London Broil. Flank Steak is called Flank Steak, and Skirt Steak is called, you guessed it...Skirt Steak. So the nomenclature is apparently slippery and regional, as often happens. But back to the steak you have on hand, yes, marinate at least a few hours, cut fairly thin across the grain, and let it rest 10 or 15 minutes post broiler/grill before slicing it. It's just not the tenderest cut, but can be very tasty.

Thank you EVERYONE who helped me find an excellent marinade for my round steak.

Why it's called "london broil" I don't know. It's a flank steak. It's not called london broil in London. I use a a korean marinade http://food52.com/recipes/13423-mysteries-of-korea-town-kalbi-style-flank-steak When I slice the steak after grilling I cut thin strips on the bias.

Here's a sample marinade although I make one with less ingredients (no balsamic): http://www.yummly.com/recipe/external/London-Broil-with-Creamy-Horseradish-Sauce-510397

If I buy London Broil (I usually look for labeled skirt or flank steak), I always marinate usually overnight or at least 4 hours in the refrigerator and broil in the oven on High or cook on the grill, weather permitting. I've read time & time again that the name refers to the type of preparation and its usually flank steak. My opinion, marinate, bring to room temperature and cook to medium-rare in a broiler pan on high. Cut thinly against the grain. I love flank and skirt steak and cook it often. If you're looking for a simple marinade, let me know.

I broil mine on a pre-heated broiler pan for around 7 minutes per side, with a simple marinade first of Worcestershire, pepper, and crushed garlic cloves. Dry the meat and salt liberally just before cooking. One of the most important things for a juicy London broil, though, is how you carve it -- you're going for no thicker than 1/2 inch slices, cut across the grain. That is very very important, as carving across the grain makes the meat much easier to chew.