In a large saucepan, heat the olive oil over medium-high. Add the diced bacon and cook, stirring occasionally, until browned and crispy, about 5 minutes; remove with a slotted spoon. Pour off all but a few tablespoons of the bacon drippings from the pan. Add the onions, mustard seed, bay leaf and ground thyme to the pan; season with salt and pepper. Lower the heat a bit and cook, stirring occasionally, until the onions caramelize and are very soft and sweet, about 20 minutes. Add the sherry vinegar and brown sugar, then stir in the bacon and simmer, stirring often, until thickened, about 5 minutes. Discard the bay leaf.
In a medium bowl, combine the beef and Worcestershire; season with kosher salt and pepper. Form 6 patties, thinner in the centers for even cooking. Preheat a cast-iron griddle or skillet over medium-high. Drizzle the patties with olive oil and cook, turning once, until browned, 7 to 8 minutes for medium-rare. During the last 2 minutes of cooking, top each patty with 2 slices of cheese. Tent the pan with foil to melt the cheese.
In a large bowl, combine the shallot, vinegar, mustard and honey. Slowly drizzle the EVOO down the side of the bowl, whisking until blended. Add the tarragon and season with salt. Add the greens and toss to coat.
Spoon lots of bacon jam on the bun bottoms; top with the sliced cornichons. Add the patties and top with the salad, some fried onions and the bun tops. Serve the burgers with the Haricots Frites.