In a pan over medium-high heat, melt the butter, then cook the onion with the baking soda, stirring occasionally, before transferring it to the slow cooker. Look for softened, golden-amber slices and a sweet, savory aroma.

Place the beef, dry onion soup mix, eggs, milk, bread crumbs, and Worcestershire sauce in a mixing bowl, then cut 4 cheese slices into chunks and add them. Small cheese pieces distribute more evenly than large folded pieces.

With clean hands, gently bring the ingredients together, form a loaf, and settle it over the onion layer. Stop mixing once everything holds together without visible streaks of egg or dry crumbs.

Empty the condensed French onion soup over the shaped loaf. The soup should coat the beef and run down around the onion bed.

Put the lid on and cook on low for 4 hours or high for 3 hours, checking that the center reaches 165°F with an instant-read thermometer. Use the center reading rather than the surface color to judge doneness.

Drape the final 2 cheese slices across the cooked loaf and replace the lid to melt them. The topping is ready when it softens and conforms to the surface of the beef.

Cut the finished loaf into portions and spoon the onion cooking juices over each serving. The slices should show scattered melted cheese pockets beneath the soft cheese topping.