Coarsely crush allspice berries with a mortar and pestle. Alternatively, place in a resealable plastic freezer bag and crush with a rolling pin or the bottom of a skillet. Transfer to a large pot. Repeat with juniper erries, then fennel seeds. Place spices in pot. Add salt, sugar, and 2 cups hot water to pot with spices. Stir until salt and sugar dissolve. Add onion, carrot, fennel, parsley and thyme sprigs, bay leaves, chiles, and cloves. Add 3 quarts very cold water and stir to combine. Place turkey breast in brine. Place a plate on top, if necessary, to keep breast submerged. Cover and chill overnight.
For roast breast:
Preheat oven to 450°F. Line a large roasting pan with foil. Set a rack inside pan. Scatter onion, sage, and thyme over rack. Remove turkey breast from brine. Pat dry with paper towels. Place turkey breast on top of herbs on rack. Brush breast with melted butter. Season with salt and pepper. Roast turkey for 30-40 min minutes, until nicely browned. Reduce oven temperature to 300°F, cover breast loosely with foil, and continue to roast until an instant-read thermometer inserted into the thickest part of breast registers 150°F, 2 1/2-3 hours total. If skin is not crispy, crank up oven to 475 F and roast 10 - 20 min. longer Take breast out of oven, tent with foil and let breast rest for at least 30 minutes before carving.