For the tomato jam: Combine the tomatoes, brown sugar and thyme sprigs in a medium skillet and place over medium-high heat. Cover and let cook for about 10 minutes. Once bubbly and jam-like, stir in the vinegar and salt. Remove from the heat and allow to cool. Discard the thyme sprigs.
Meanwhile, for the fonduta: Fill a medium saucepan halfway with water and bring to a boil over high heat. Lower the heat so the water just simmers. Combine the fontina, milk and cream in a medium heatproof bowl. Place the bowl over the simmering water, making sure the bottom is not touching the water. Cook, stirring occasionally, until the cheese is melted. Increase the heat and whisk in the yolks and butter. Cook, whisking constantly, until the mixture is glossy, about 8 minutes. Season with salt and pepper.
For the burger patties: Mix the pork and beef in a large bowl until combined. Divide into four 4-ounce patties, each about 1 inch wider than the buns. Season both sides with salt and pepper. Set the patties on a plate or sheet pan.
Place the pancetta on the foil-lined sheet pan. Broil until crispy, 4 to 5 minutes. Set aside.
Heat a griddle or large nonstick skillet over high heat. Add the oil. Add the patties and cook until charred, 2 to 3 minutes on one side, flip, and cook another minute on the other side, or until the patties reach the desired doneness. Transfer the patties to the bottom halves of the potato buns. Top with some tomato jam, pancetta, fonduta and arugula. Cover with the top buns. Smash down and take a bite!