Step 1Fry the bacon in a hot skillet until done but not too crisp, about 8 minutes. Drain on paper towels and set aside.
Step 2Cut the avocados into quarters or sixths. Sprinkle lightly with lemon juice, add the chives and toss. Season with sea salt and pepper. Set aside in a serving dish.
Step 3Cut the tomato into slices about one-fourth-inch thick and season lightly with sea salt and pepper. Set aside. Cut the red onion into very thin (about one-eighth-inch) slices and season lightly with sea salt and pepper. Set aside.
Step 4Crumble the blue cheese and cheddar into separate serving bowls. Slice the Gruyere with a cheese plane onto a serving plate and set aside.
Step 1Brush the cut side of each bun with olive oil and set aside. Light an outdoor grill or place a seasoned cast-iron skillet over high heat.
Step 2Being careful not to overwork the meat, shape it into 6 patties, about 2 inches thick and 4 inches in diameter. Sprinkle each side of each patty with about one-fourth teaspoon of kosher salt and 4 to 6 grindings of freshly ground black pepper.
Step 3Place the burgers on the hot grill or in the hot cast-iron skillet. Sear them until they can be turned without sticking, then turn them. For rare, cook about 4 minutes, then flip and cook another 4 minutes (for medium-rare, 5 minutes on each side; for medium, 6 minutes).
Step 4As the burgers are cooking, toast the buns, cut side on grill or pan, for a minute or two. Just before the burgers are done cooking, garnish with a little good-quality sea salt, then top with cheese and let it melt. Have guests place lettuce, onion, tomato, bacon and avocado on the bottom half of their toasted bun. Place the burger on top, then the top of the bun. Serve ketchup, mayonnaise, mustard, pepperoncini and dill pickles with burgers.