Heat a large skillet over medium-high heat. Add the ground beef and break it into small crumbles as it cooks, stirring occasionally for about 8-10 minutes until the meat is deeply browned and no longer pink. This browning step develops rich, savory flavor through the Maillard reaction—don't rush it or stir too frequently. Once browned, add the diced onion to the pan and cook for another 2-3 minutes until the onion softens slightly and releases its aroma.

Tilt the skillet and carefully pour off the excess fat and grease into a bowl or sink, leaving just a light coating on the meat. Transfer the browned beef and onion mixture to your slow cooker, scraping the pan to capture any flavorful browned bits stuck to the bottom.

In a medium bowl, whisk together the flour, chili powder, garlic powder, and smoked paprika until well combined and no lumps remain. This prevents the flour from clumping when it hits the liquid in the slow cooker. Then add the tomato sauce, ketchup, water, brown sugar, apple cider vinegar, yellow mustard, Worcestershire sauce, and optional hot sauce to the bowl, stirring until you have a smooth, well-blended sauce. I like to taste the sauce at this point and adjust the sweetness or tanginess to my preference before it goes in the slow cooker—it's much easier to adjust now than after cooking for hours.

Pour the sauce mixture from Step 3 over the beef in the slow cooker and stir well, making sure the meat is evenly coated and the flour is fully incorporated. Season with salt and black pepper, stirring to combine. Cover the slow cooker and cook on low for 4-6 hours, stirring occasionally if you're home—the longer cook time develops deeper, more complex flavors. The mixture should be slightly thick but still saucy when done; if it seems too thin at the end, you can leave the lid off for the final 30 minutes to let some liquid evaporate.

While the sloppy joes finish cooking, butter the hamburger buns lightly and toast them in a skillet over medium heat or on a toaster oven rack until they're golden and crispy—this prevents them from getting soggy when filled. Taste the sloppy joe mixture and adjust seasoning with additional salt, pepper, or a splash of vinegar if needed. Spoon the meat mixture generously onto each toasted bun and serve immediately.