Buy the right meat. For juicy burgers, get ground chuck with a fat content of at least 18%. Lean and extra-lean meat make tough, dry burgers. Also, the more freshly ground the meat is, the more tender and flavorful the burger: If your store has butchers, ask them to grind the meat fresh for you. (Or just grind your own, following our no-fuss method, see below.)
Mix in your seasonings very, very gently. The more you handle the meat, the tougher your burger will be. In a large bowl, pull the meat apart into small chunks, add egg yolks, gherkins, shallots, capers, peppercorns if using, and salt and toss gently with fingers spread apart until loosely mixed.
Use wet hands to form patties. This keeps your hands from getting sticky. It also allows the meat to come together faster and prevents overhandling.
Make patties thinner in the center. Divide the meat into 4 equal portions and form patties about 3/4 inch thick at the edges and 1/2 inch thick in the center. They'll shrink and even out when cooking.
Keep meat cold until it goes on the grill. Put the patties in the fridge while the grill heats up. This helps more of the flavor-carrying fat stay in the meat.
Use a clean, well-oiled, preheated grill. Bits of debris encourage sticking, as does an unoiled surface and too low a temperature; you want your burgers to quickly sizzle, firm up, and release from the grill.
Keep grill at a steady high heat (you can hold your hand 1 to 2 inches above grill level for 2 to 3 seconds). If using charcoal, you want ash-covered coals to produce even heat. With a gas grill, keep the lid down while cooking; with a charcoal grill, leave the lid off.
Flip burgers once and at the right time. Constant turning will toughen and dry out meat, and if you flip too soon, burgers will stick. Cook 2 minutes per side for rare, 3 for medium-rare, 4 for medium, and 5 for well-done.
Don't press on the burgers while they're cooking. The juice that seeps out holds most of the flavor and moisture.
Let burgers rest a few minutes before eating. This allows them to finish cooking and allows their juices, which have collected on the surface during grilling, to redistribute throughout patty.
The real secret: Grind your own meat.
Grinding meat at home is not only easier than most people think, but also makes the moistest and most flavorful burgers. And, given the periodic safety concerns about commercially ground meat, home-ground is the way to go if you like your burgers cooked rare or medium. Manual meat grinders (about $30) are available at kitchen supply stores, and grinder attachments (about $50) for standing mixers work very well.
For four 6-ounce burgers, buy 1 1/2 pounds chuck roast or sirloin, keeping a thin layer of fat on the meat.
For added safety, bring a large pot of water to a boil and boil the roast for 30 to 60 seconds. Remove meat and rinse with cold water.
Cut the meat into 1-inch pieces. In a large bowl, toss meat pieces with 1 teaspoon salt. Cover and refrigerate overnight.
Chill the grinder for 30 minutes before starting (a cold grinder grinds more efficiently).
Set up grinder according to manufacturer's instructions, using the coarse plate or setting. Feed meat into funnel and grind, stopping to clear the grinder if necessary. Put ground meat through grinder once more and proceed with step 2 above.
Note: Nutritional analysis is per serving.