Season beef with salt and pepper. In a Dutch oven or heavy deep skillet, heat half of oil over medium-high heat – brown beef all over, turning with tongs. Transfer to slow cooker. Reduce stove-top heat to medium. Add remaining oil to pan, along with onions and garlic. Cook, stirring occasionally, until softened and golden, about 4 minutes. Transfer to slow cooker. Increase stove-top heat to high. Add tomatoes, soup mix, tomato paste, beer, chili powder, cumin and cayenne. Scrape up any brown bits with wooden spoon. Bring to a boil, transfer to slow cooker. Cover and cook on low for 8 to 10 hours (or high for 4 to 5 hours) or until meat is tender. Transfer meat to a cutting board – tent with foil and let stand for 10 minutes before pulling into shreds with a large fork. Cover and keep warm. Meanwhile, skim fat from liquid in slow cooker. Whisk flour with 1/4 cup water – whisk into slow cooker. Cover and cook on High, stirring once, until thickened, about 15 minutes. Serve sauce with beef and sprinkle with green onions. Use to top mashed or baked potatoes. Set aside half of the shredded meat and sauce.