For the brine: Bring 2 quarts water to a simmer in a stockpot over medium-high heat. Add the salt, brown sugar, vinegar, peppercorns, pepper flakes, parsley, thyme, garlic, bay leaves, lemon peel and onions. Heat until the salt dissolves, 7 to 8 minutes. Remove from the heat and stir in 4 cups ice cubes, stirring until the ice melts. Chill the brine for 15 to 20 minutes before using. (The brine keeps in the fridge for up to 2 days.)
For the turkey picnic sandwich: Place the turkey breast in the chilled brine and refrigerate for 8 hours or overnight.
Preheat the oven to 350 degrees F. Remove the turkey from the brine and pat dry. In a small bowl, add the olive oil, salt, pepper, thyme and garlic and mix well to combine. Rub the turkey all over with the herb oil mixture.
Roast in the center of the oven until an instant-read thermometer inserted into the thickest part of the breast (avoid touching bone) reaches 165 degrees F, about 1 hour. Remove the turkey from the oven and allow to rest for 20 minutes before carving into thin slices.
For the sandwiches: Smear the bottoms of the pretzel rolls with mustard and mayonnaise. Working from the bottom up, layer the sandwich with a pile of sliced turkey (pile it high!), some Cranberry-Onion Jam, a pickle, a pile of romaine and radicchio, and finally a slice provolone and Swiss cheese. Place top half of the rolls on top and secure with sandwich picks.
Set a medium saucepan over medium heat and add the oil and onions. Cook, stirring often, until soft and light brown, about 10 minutes. Add the cranberries, orange juice, sugar, salt, zest, pepper flakes, rosemary and 1/4 cup water. Bring to a simmer and cook for 10 minutes. Reduce the heat to low, remove the rosemary, and continue to cook until the liquid has reduced and you have a jam-like consistency, about 10 minutes more. Let cool.