Place the roast and onion in gallon bag. Sprinkle in cumin, then add salsa, and rotfl. Zip close and freeze. When ready to thaw, thaw in fridge for 24 hours. Empty into crockpot for 8-10 hours. Once cooked, remove roast from crockpot and shred. Drain juice out of crockpot, reserving some in a bowl. Leave tomatoes and onion in crockpot. Add meat back in crockpot, stir. Add some of the reserved juice back. Serve on tortillas with desired toppings.