Prepare steak: Place garlic, salt, pepper and thyme on a cutting surface and use a large knife to chop together until mixed. Transfer to a large ziplock bag. Add soy sauce and oil; seal and press bag gently to combine ingredients. Place steak in bag and seal. Turn several times to coat meat. Refrigerate, turning several times, for at least 3 hours and up to overnight.
Prepare sauce: Beat cream, sour cream, mustard, lemon juice and horseradish with an electric mixer until mixture forms stiff peaks. Season with salt and pepper. Pour into a chilled serving bowl, cover and refrigerate. (Sauce can be made up to an hour before serving.)
Preheat broiler to high and arrange rack 6 inches from heat source. Remove meat from marinade and pat dry; discard marinade. Broil on a broiling pan for 7 minutes. Turn meat over and broil 5 to 7 minutes longer. To check for doneness, test with the tip of a sharp knife. If juices run red, broil 1 or 2 minutes longer, until an instant-read thermometer reads 140°F (for medium-rare). Let steak rest on a cutting surface for 10 minutes. Working against the grain, cut into thin slices with a sharp knife. Serve with horseradish sauce on side.
I made this for dinner guests and followed the recipe exactly as written with just a couple changes: 1. I used 3.55 lbs tri tip steaks (4 of them) instead of London broil 2. I used an entire 4 oz jar of hot horseradish for the sauce (I kept tasting it and couldn't taste the horseradish really until I added the whole bottle - wasn't very hot but tasted really good)I will make this again and again. It was amazing. I worried that the soy sauce and horseradish flavors would clash, but they didn't. I marinated the steaks for about 3 hours in a ziplock bag in the fridge, grilled for about 4-5 minutes per side, and they tasted great without the sauce, but the sauce was to die for. I could eat it straight! Our guests loved it, too. One doesn't normally like horseradish but loved this sauce, and another was dipping everything in it, and my husband had to force himself to stop eating. One note - I made the sauce in a 16 oz wide-mouth mason jar using my immersion blender with the whisk attachment, and it pretty much filled the whole jar once the whipping cream kicked in. It's fine with me because I'll figure out other uses for it, but just a little warning. I served this with asparagus spears that I tossed with olive oil, sea salt, and a pinch of red pepper flakes, grilled, then finished with a squeeze of fresh lemon. And to not leave out the carbs, I sliced fresh Italian bread, brushed both sides with olive oil that I mixed with crushed garlic, sea salt and pepper, and grilled on both sides for just a minute or two to toast. It really was the perfect summer meal. Enjoy!PS - I thinly sliced the leftover steak and tossed with rice and added the horseradish sauce and had it for lunch the next day. It was divine!