Wipe the meat dry, then rub it all over with salt and pepper. Heat a nonreactive skillet over a medium-high flame, add 2 tablespoons of the fat and swirl to coat. Add the beef and brown it on all sides, about 4 minutes per side. Remove it from the pan and place it on the bottom of a slow cooker.
Lower the heat under the skillet slightly, then add the remaining fat. Once it melts, add the onions. They should sizzle as they hit the pan. If they sputter or splatter, lower the flame. Stir well, coating the onions in the cooking fat and scraping up any browned bits from the bottom of the pan. Continue to cook 2-3 minutes longer until the onions have given up two thirds of their volume in water. Once they start to brown, lower the flame and allow them to caramelize by cooking 15-20 minutes longer, stirring often. Lower the heat if they start to burn.
Pour the onions over the top of the beef in the slow cooker. Return to the pan to the flame and add the coffee, cinnamon stick, and honey (preferably buckwheat!). Whisk to blend, scraping up any browned bits and incorporating them into the sauce. Bring it to a boil and cook until it is reduced by half. Pour it over the meat, adding the cinnamon stick to the slow cooker as well. Add the orange zest, cover and cook on low for 6-8 hours until tender. Serve the meat with the onions and sauce spooned over the top.