3-4 lb boneless chuck roast
1 onion, minced
1 jar salsa
1 small can diced green chilies
juice of 1-2 limes
2-3 packs of taco seasoning

Brown chuck roast in a large skillet with a little olive oil. Brown on all sides. Put the roast and the onions in a slow cooker.
Mix the salsa, chilies, lime juice and taco seasoning in a bowl. Pour over the roast in the slow cooker. Cook on low for 6 hours, until fork tender.
 When the roast is done, remove it to a cutting board. Shred. Put the liquid in a saucepan and reduce by half.  Return the meat to the slow cooker and mix in the reduced liquid. 
Use this to fill the tacos.
Other fixins': shredded lettuce, red onion, diced tomatoes, diced black olives, avocado, spicy coleslaw,. sour cream...