In a small bowl whisk together garlic, brown sugar, vinegar, soy sauce, cornstarch and rosemary.
Rub olive oil over frozen pork shoulder. Pour broth into pot.
Place frozen roast in pressure cooker, pour balsamic vinegar mixture on top of meat.
Close lid and seal steam valve. Press meat/stew button and cook for 1 hour 45 minutes or 22 minutes per pound you are using.
When done allow to naturally release steam.
Lift lid and slice or shred and then put meat back into pot with sauce to coat. If you want a thicker sauce leave meat out. Set pot to saute and add 1/2 c of your hot liquid into a bowl with 2 tbsp cornstarch.
Whisk until smooth and add into pot. Stir and allow to bubble and thicken, then turn pot off and put shredded meat into thick sauce, serve.