Brown chicken quickly in a skillet with oil for extra flavor, then transfer to the crockpot.
Add tomatoes, broth, taco seasoning, and onion over the chicken. Stir gently.
Cook on low 6–7 hours or high 3–4 hours, until chicken shreds easily and the thickest piece reaches 165°F.
Shred in the pot, stir and taste; add salt if needed. Cool slightly, portion into freezer bags, flatten, label, and freeze.
Reheat thawed portions in a saucepan, microwave, or oven until the center reaches 165°F. If reheating from frozen, use the microwave, oven, or stovetop and verify 165°F; use a slow cooker only to hold already-hot food at 140°F or above.