Place kosher salt, whole peppercorns, fresh thyme leaves and garlic cloves in a large stock pot (large enough to cover your turkey breast with a lid on). Then pour in warm water. Whisk water until salt has dissolved (about 2 minutes). Pour the buttermilk into the pot. Whisk again then place the turkey inside the pot. Turn the turkey breast so that only a small piece is exposed. Place lid on pot and refrigerate for at least 12 hours no more than 24 hours.
After turkey breast has sat in brine for at least 12 hours. Rinse turkey breast off thoroughly and pat dry before baking.