Heat 2 Tbsp olive oil in a large skillet over medium high heat until it begins to shimmer, add the chuck roast and brown 3–4 minutes per side. When the roast is browned on both sides, transfer it to the slow cooker.
Add the additional 2 Tbsp of olive oil to the skillet if needed and add the onions. Cook 3–4 minutes, scraping up any brown bits that are stuck to the bottom of the skillet. Add garlic, cook 1 additional minute then add tomato paste, chili powder, oregano, red chili flakes and cumin. Stir into a paste, cook approximately 30 seconds to toast the spices, then add the puréed tomatoes, beef bouillon and chicken broth. Bring to a boil, scraping the bottom of the pan, remove from heat then pour over the roast.
Cover and cook on low heat 8–9 hours or 5–6 hours on high heat until it falls apart when pierced with a fork.
Remove the meat from sauce, shred it into ¼–½" pieces then place it into a large bowl and cover with aluminum foil to keep warm. Remove the fat from the top of the sauce with a skimmer, paper towel or fat separator.
Pour sauce into a skillet, add remaining spices and bring to a bubble. In a small bowl or liquid measuring cup, add cornstarch to ¼ cup water. Whisk until combined then pour into sauce mixture. The heat will help the starch thicken and become glossy. Stir constantly and cook for 2–3 minutes until the sauce thickens.
Add meat to skillet and coat with sauce mixture. Return to slow cooker and adjust the temperature to low to keep warm. Now sit back and relax while your guests serve themselves on slider buns garnished with coleslaw and pickles.
One Last Thing
Replace the buns with tortillas and BOOM—beef tacos!