The slow-cooker is great to use all year-round. Use it in summer to help keep your kitchen cool and ensure that you are out of the kitchen enjoying summer-fun! If you find Ancho chili paste in the grocery store, you can substitute it for the canned chipotle peppers.
Season roast all over with salt and pepper. Brown in lightly oiled frypan over medium high heat. Remove from heat; set aside.
Add onion, tomatoes, beer and chili powder to slow cooker insert.
Rub roast all over with minced chipotle peppers and add to slow cooker; add in enough broth to come ¾ of the way up the side of the roast. Cover and slow cook on low power for 10 to 12 hours.
Remove cooked roast to large bowl, removing twine and any external fat; set aside.
Skim fat from surface of sauce. Transfer to a saucepan; cook over medium-high heat until reduced by about 1/3. Stir in tomato paste and heat through. In small cup combine cornstarch with an equal amount cold water until smooth; gradually stir into tomato sauce and bring to a boil to thicken. Meanwhile use 2 forks to pull the beef apart (shred); add sauce to the meat. Season with salt and pepper to taste if necessary. Serve over toasted slider burger buns or baked potato.