Brown the beef. Pat the beef dry with paper towels, keeping the pieces similar in size. Heat the butter and oil in a large heavy-bottomed pot over medium-high heat. Add the beef in a single layer and let it develop a deep golden-brown crust before turning. Work in batches if needed, transferring browned pieces to a plate. Lower the heat slightly if the butter starts getting too dark.
Soften the onions. Return all the beef and any collected juices to the pot. Reduce the heat to medium and add the chopped onions. Cook for about 5 minutes, stirring occasionally, until the onions begin to soften.
Add the potatoes and seasonings. Add the 5 crushed garlic cloves and potatoes, followed by the salt, black pepper, cumin, coriander, rosemary, paprika, and turmeric. Cook for about 5 minutes, stirring gently to coat everything in the seasonings. Keep the heat moderate so the garlic and spices do not scorch.
Stir in the carrots and bell pepper. Add the sliced carrots and bell pepper strips. Cook for another 4–5 minutes, stirring occasionally, until they begin to soften.
Add the tomatoes and broth. Stir in the tomato paste and crushed tomatoes, then pour in the unsalted beef broth. Gently scrape up any browned bits from the bottom of the pot and bring the sauce to a gentle simmer.
Simmer until tender. Cover and reduce the heat to low, keeping the sauce at a gentle bubble. Simmer for about 90 minutes, checking occasionally and stirring gently to prevent sticking. The beef should yield easily to a fork, and the potatoes should be tender all the way through. Continue cooking if needed, adding a splash of broth or water if the sauce gets too thick. If the vegetables are ready before the beef, gently lift them out and return them once the meat is tender.
Add the finishing touches. Stir in the sugar and turn off the heat. Mix the chopped parsley with the 4 finely chopped garlic cloves and scatter over the stew. Cover and let it rest for 10 minutes before serving.
Serve. Spoon the stew into bowls with plenty of the tomato sauce. Serve on its own with bread, or alongside mashed potatoes.