In a large skillet, or in your slow cooker if it has a browning feature, heat olive oil over medium high heat. Generously season the chuck roast pieces with with salt and pepper. Brown all sides of the meat working in small batches and then setting aside. (This step can be omitted, but we feel it contributes to the wonderful texture of the meat.)
Using the same skillet, brown the ground beef. Drain off the fat into a heat proof bowl.
Place the browned chuck roast and ground beef in your slow cooker along with all other ingredients and cook for 4 hours on high, or 8 hours on low.
Once finished, stir, breaking up any large pieces of ground beef and adjust the salt and pepper to your liking.
Serve with slivered jalapeños, avocados, cilantro and green onions. Enjoy!