Season the pork pieces with Essence on all sides. In the pressure cooker, heat the oil over medium-high heat. Add the pork, in batches if necessary, and cook until just browned, about 2 minutes per side. Remove to a plate and set aside.
Add the onions and saut� until just tender, about 2 minutes. Add the stock and beans and stir well, scraping up any browned bits off the bottom of the pot. If the brown bits are not released from the bottom of the pan, they could stick and burn and impart a burnt flavor to the final dish.
Place the lid on the pressure and lock into place. Bring the pressure cooker to high pressure. Lower the heat just enough to maintain high pressure and cook for 15 minutes. Turn off the heat and allow to come to normal pressure naturally - this could take up to 10 minutes. Once the pressure is completely released, remove the lid, tilting away from you, to allow excess steam to escape.
Add the salt pork, tomatoes, garlic, light brown sugar, mustard, chili powder, bay leaf, thyme, black pepper, and reserved pork and cover. Lock the lid into place and bring the pressure cooker to high pressure. Lower the heat just enough to maintain high pressure and cook for 15 minutes.
Remove from the heat and allow to come to normal pressure naturally - this could take up to 10 minutes. Once the pressure is completely released, remove the lid, tilting away from you, to allow excess steam to escape.
Bring to a simmer and season with salt and pepper. If the meat or beans are not tender enough, cover and simmer for 15 more minutes, or lock the lid, bring back to high pressure and cook over high pressure for another 2 to 3 minutes. Discard the bay leaf and thyme sprig, and serve.