Heat a skillet over medium-high heat and brown the ground beef for about 8 to 10 minutes, breaking it apart as it cooks. Once fully cooked, drain any excess grease and set the meat aside.
In a large bowl, dissolve the gelatin in water and let it sit while preparing the stock.
In a separate pot over high heat, sauté the minced onion, celery, salt, and pepper for a minute before adding the beef stock. Bring to a boil and let it cook for 3 minutes.
Strain the hot stock into the gelatin mixture, stirring thoroughly to ensure even distribution.
Allow the mixture to cool slightly and begin thickening but not fully set (this may take around 40 minutes to an hour). Once it reaches the right consistency, fold in the cooked ground beef and pimiento peppers.
Pour the mixture into ramekins, individual molds, or a loaf pan. Refrigerate overnight until fully set.
To unmold, briefly dip the container in warm water (taking care not to let water seep in) for 10 to 20 seconds, then turn it onto a serving plate. If needed, use a thin knife to loosen the sides. Serve chilled, traditionally atop a bed of lettuce.