Before you slow cook it, make a paste of cumin, black pepper, olive oil garlic and paprika, rub the meat, and let it sit overnight (this is why the good lord invented freezer-sized zip lock bags, JMO) in the fridge. That'll give it time to soften/flavor anything freezer burned on the surface. Save the soy and/or salt for the liquid portion of the cooking, unless you want to invest the time to brine it (which seems like a waste if you're going to slow cook).