Place the meat pieces and the whole yellow onion in a large pot. Add enough water to cover the meat by about 1 inch. Bring to a boil over high heat, skim off any floating scum and fat, then reduce heat and simmer 10–15 minutes. Turn off the heat. Remove the meat and the whole onion from the pot and set them aside. Reserve some of the cooking broth for later.
Heat the 2 tablespoons vegetable oil in a large skillet over medium heat. Add the trimmed, cut green beans and sauté 4–5 minutes, until bright green and slightly tender. Transfer the beans to the slow cooker (or set aside if using another cooking method).
Into the slow cooker add the parboiled meat, the whole onion you removed earlier, the sautéed green beans, 1/2 teaspoon salt, 1/4 teaspoon ground black pepper, the bay leaf, and 1/2 teaspoon baharat. Cover and cook on high for 4–4.5 hours or on low for 8 hours.
When the meat and beans are almost done (about 20–30 minutes before the end of the cooking time), use the same skillet (no need to clean it first) to make the tomato-thickening mixture: over medium heat sauté the chopped yellow onion for about 2 minutes, then add the minced garlic and cook 1 minute more.
Stir the 2 tablespoons all-purpose flour into the onion and garlic, then add the 3–4 tablespoons tomato paste and mix until combined. Gradually add some of the reserved broth, stirring until you have a smooth sauce. Bring the mixture briefly to a boil, then remove from the heat.
Pour the tomato-broth mixture into the slow cooker, stir gently to combine, cover, and cook an additional 20 minutes to allow the sauce to thicken and flavors to meld.
Remove the bay leaf, taste and adjust seasoning if needed, and serve the stew warm.