Place the roast and onion in gallon-sized freezer bag. Sprinkle in cumin, then add salsa & Ro-Tel. Zip close & place in freezer. When ready to use thaw in fridge for 24 hours. Empty contents into crock-pot and cook on low 8-10 hours.
Once cooked, remove roast from crock pot and shred
Drain juice out of crock pot, reserving some in a small bowl. Leave onions & tomatoes in crock pot.