Heat the bacon grease in a large heavy-bottom pot over high heat. When hot, add the beef and sear until well browned. Use tongs to rotate the beef and brown on all sides, about 8 minutes. Transfer all the beef to a plate, leaving any fat in the pot.

Add the onion and bell pepper along with a generous pinch of salt and cook over medium-high heat, stirring often until the onions are soft and translucent, about 5 minutes. Add the garlic and cook until fragrant, about 1 minute more. Add the tomato paste and cook another 2 minutes. Stir in the paprika, cayenne, bay leaf, fish sauce and cook for 1 minute more. Add the stock and gelatin and stir to combine everything well.

Add the beef back to the pot and bring the liquid to a boil. The meat should be completely submerged so add a bit more chicken stock, if necessary. Reduce the heat to a simmer and cook, covered on low for 3 hours, or until the meat is completely tender (check at 2 1/2 hours).

If there is a lot of liquid, uncover the dish and simmer for another 20-30 minutes to reduce the broth to desired consistency. Taste and adjust the seasoning with salt and pepper, to taste. Stir in the parsley and serve with Greek yogurt or sour cream, if desired.