Prepare the brine by combining the brown sugar and salt with 2 cups of warm water, stirring until salt and sugar are dissolved. Add 2 cloves of minced garlic, one rosemary sprig and the pork roast to a large zip-to-lock bag. Refrigerate for 1 ½ hours.
Remove roast meat from refrigerator 30 minutes before ready to cook. Preheat oven to 325℉.
Remove the remaining rosemary leaves from stems and mince with the remaining garlic cloves (or crush with mortar and pestle). Combine with olive oil to make a rub.
Heat a large cast iron skillet (9-10 inch) on medium high heat about 5 minutes
Remove meat from brine and dry with paper towel. Rub with crushed garlic/rosemary mix.
Grill one side for about 5 minutes until brown (if your roast has a "fat side" grill that!).
Flip roast and place in oven. Roast in 325℉ oven for 30-40 minutes until it reaches 140℉ internal temperature.
Remove meat from pan and tent with aluminum foil for 15 minutes. Cut into ¼ to ½ inch slices.