In a large bowl, add the onions, the garlic, the olive oil, the vinegar and the parsley and stir everything together. Add the roast to that mixture and let it marinate overnight.

The next day, in a large skillet, add a few tablespoons of olive oil and brown the roast on all sides.

Transfer the roast to the pressure cooker and add water (enough to cover the half the roast).

Close the pressure cooker and let it cook on high heat until you hear the pressure.

Lower the heat to medium and cook for 40 minutes or until the meat is tender.

In the same skillet you browned the roast, add the onions and garlic from the marinade and sauté them until almost browning. Add the liquid and let it reduce a little bit.

Once the roast is tender, add the "gravy" and the potatoes to the pressure cooker and cook everything - uncovered (without pressure) - for about 20 minutes, or until the potatoes are cooked and the gravy has reduced by half and is this beautiful dark brown color.(Remove the potatoes beforehand if they are cooked before the gravy is ready).

Serve the roast with the gravy on top and the potatoes on the side.