Heat the oil in a large skillet over medium until the oil shimmers. Add half of the beef, and saute until browned. Transfer slow cooker. Add the remaining beef to the pan and brown. Transfer that batch to the slow cooker. Add 2 cups of the beef broth to the slow cooker (refrigerate the remainder).
With the pan still on medium heat, deglaze with some of the beer or red wine (I find beer works better). Add the onions, and stir to coat in the saucy fond. Cook for 5 minutes, or until translucent. Add the tomato paste to the pan and mix well with the onions, cooking until the tomato paste just begins to stick to the pan (a few minutes). The onion mixture should be thick and saucy; if it's at all watery, sprinkle 1 teaspoon of flour over the onions and mix in.
Transfer the onion mixture to the slow cooker, along with the beer, red wine, Worcestershire sauce, and bay leaves. Cover with the lid.