Heat the vegetable oil in a large skillet over medium-high heat.
Add the beef cubes in batches and brown them on all sides, about 5 minutes per batch. Transfer browned beef to the slow cooker.
In the same skillet, add the chopped onion and cook until softened, about 4 minutes.
Add the minced garlic and cook for 1 minute more, stirring constantly to prevent burning.
Stir in the sweet paprika, smoked paprika, dried marjoram, black pepper, salt, and cayenne pepper. Cook for 1 minute to release the spices’ aroma.
Add the diced tomatoes with their juice, tomato sauce, tomato paste, and beef broth to the skillet. Stir well to combine and bring to a simmer.
Pour the tomato mixture over the browned beef in the slow cooker.
Add the chopped bell peppers and bay leaf to the slow cooker. Stir gently to combine all ingredients.
Cover and cook on low for 6 hours, or until the beef is tender.
About 30 minutes before serving, mix the all-purpose flour with 1/4 cup water in a small bowl until smooth.
Stir the flour mixture into the slow cooker to thicken the sauce.
Cover and cook on high for an additional 30 minutes.
Remove the bay leaf before serving.
Serve hot over egg noodles, mashed potatoes, or rice.