Brown beef: Heat 1 tablespoon of the canola oil in a large skillet over medium-high. Add half of the beef to skillet. Cook, turning occasionally, until well browned on all sides, 6 to 7 minutes. Transfer browned beef to a 6-quart slow cooker. Repeat with remaining oil and beef.
Prepare cilantro, onion, and peppers: Coarsely chop cilantro leaves, and set aside. Finely chop cilantro stems. Add chopped onion and poblanos, smashed garlic, and cilantro stems to skillet. Cook onion mixture over medium-high, stirring occasionally, until tender, about 5 minutes.
Cook spice mixture: Add all-purpose flour, cumin, chili powder, and oregano to skillet, and stir until combined. Cook, stirring often, about 1 minute. Add beer to skillet, and cook about 2 minutes, stirring and scraping to loosen browned bits from bottom of skillet.
Add stock mixture to slow cooker: Stir in stock, fire-roasted tomatoes, brown sugar, salt, and pepper. Let the stock mixture come to a boil. Remove skillet from heat, and add stock mixture to the browned beef in the slow cooker. Stir to combine.
Cook on low: Cover slow cooker; cook beef-stock mixture on LOW until beef is tender, about 6 hours (or cook on HIGH for 3 hours and 30 minutes). Ladle stew into bowls, and sprinkle each serving with reserved chopped cilantro leaves. Serve with warm flour tortillas on the side.
