In a small bowl, toss the onion, vinegar and sugar; season with salt and pepper and let pickle 15 minutes.
Position a rack in the center of the oven and preheat to 400 degrees . In a food processor, combine the lemon juice, tahini, EVOO, rosemary and garlic. Process until smooth, adding bottled or filtered water by the tablespoon to thin the dressing, if needed. In a large bowl, toss the kale, carrot and dressing; season.
Heat a cast-iron or other ovenproof skillet over medium.
In a medium bowl, mix the cheeses, creme fraiche and mustard.
Thinly coat one side of each bread slice with butter. Place on a work surface, buttered side down. Slather the bread with the cheese mixture. Place the bread in the skillet, cheese side up, and cook until the bottom browns, 2 to 3 minutes. Transfer to the oven and bake until the cheese is bubbly and browned, 7 to 10 minutes. Top with some of the drained pickled onions. Serve with the salad.