1. Heat a large skillet over medium-high heat, add ground beef, and cook, breaking up meat with the back of a spoon, until meat begins to fry, 12–15 minutes. Drain meat in a colander, then return it to the skillet over medium-high heat.
2. Add chicken stock, season to taste with salt and pepper, and cook, stirring often, until liquid is absorbed and meat is well browned, 15–20 minutes.
3. Divide meat between hamburger bun bottoms, spoon some onion on each, and put other bun half on top.

Serves 8.