In a large pot, combine 1 gallon of water, 1 cup of kosher salt, and 1 cup of brown sugar.
Add black peppercorns, dried thyme, dried rosemary, crushed garlic, and allspice berries to the pot.
Heat the mixture over medium heat, stirring until the salt and sugar have fully dissolved.
Remove the pot from heat and let the brine cool to room temperature. If using lemon, add the slices now.
Once cool, pour the brine into your brining container (cooler or resealable bag).
Submerge the turkey breast side down in the brine, ensuring it is fully covered. If needed, add ice to keep it chilled.
Seal the container and refrigerate or store in a cool place for 12 to 24 hours.
After brining, remove the turkey, rinse with cold water, and pat dry with paper towels before cooking.