Make the beans: pour the rinsed beans into a medium mixing bowl and cover with cold water. Let sit for about 1-2 hours until water is absorbed into the beans (the skin on the beans will look a little wrinkly). Drain the beans and put in a slow cooker. Cover with 4 cups water, bay leaf, oregano and salt and cook on high for 2 hours. Taste the beans -- they should be slightly al dente but cooked through (not sandy or gritty tasting). Remove the beans from the slow cooker, throw out the bay leaf and drain the beans. Set aside.
Make the chili: in your stovetop-safe slow cooker insert or a heavy-bottomed skillet, heat 2 tablespoons olive oil over medium-high heat until the oil is shimmering. Place the chicken thighs into the skillet/slow cooker insert, bone-side up, and brown the chicken thighs on all sides (about 1-2 minutes per side). Remove the chicken thighs and put on a plate.
Reduce the heat to medium and add diced onion and garlic. Saute for 1-2 minutes, then add the chili powder, cumin, oregano, fennel seeds, 1 teaspoon salt and pepper. Reduce the heat to medium low and continue sauteing the onion mixture until the onions are soft and translucent, about 7-8 minutes. Add the flour and stir to combine (the mixture will be clumpy, but that's okay.
Transfer the onion mixture to your slow cooker. Slowly pour in the chicken broth, stirring continuously to mix together the onion mixture and the broth. Add the diced tomatoes and chicken thighs, pushing the chicken thighs down into the broth to make sure it's fully covered.
Add 3 cups of the cannelini beans (if you're using canned beans, reserve one cup of cannelini beans). Cover the slow cooker and set the slow cooker to cook on low for 8 hours.
About an hour before serving (and with about an hour to go on the slow cooker), remove the chicken thighs from the slow cooker and set on a plate to cool slightly. Mash (or puree in a food processor) the reserved cannelini beans (if using canned; otherwise, use about a cup of the cooked dried cannelini beans). Add the mashed/pureed beans along with the broccoli rabe back to the slow cooker and stir to incorporate. Shred the meat from the chicken thighs with two forks and return the chicken meat to the slow cooker. Cover and cook for the remaining hour, making sure that the rabe is sufficiently tender before serving.
Serve immediately with suggested toppings.