Pat the beef cubes dry with paper towels and season generously with salt and pepper.

In a large, heavy-bottomed pot or Dutch oven, heat the olive oil over medium-high heat. Add the beef in a single layer, being careful not to overcrowd. Brown on all sides, about 3-4 minutes per side. Remove and set aside. Repeat with remaining beef, adding more oil if needed.

Add the chopped onions to the pot and cook over medium heat, stirring occasionally, until softened and translucent, about 5-7 minutes.

Add the minced garlic, sweet paprika, smoked paprika, caraway seeds, marjoram, and cayenne pepper (if using) to the pot. Cook for about 1 minute, stirring constantly, until fragrant.

Add the tomato paste to the pot and cook for another minute, stirring constantly.

Pour in the beef broth and red wine (if using) and scrape the bottom of the pot to loosen any browned bits.

Return the browned beef to the pot. Add the diced tomatoes (with their juice), red bell pepper, and yellow bell pepper.

Bring the mixture to a simmer, then reduce the heat to low, cover the pot, and simmer for at least 2-3 hours, or until the beef is very tender. Check occasionally and add more beef broth if needed to keep the beef covered in liquid.

After the goulash has simmered for at least 2 hours, add the cubed potatoes to the pot. Continue to simmer, covered, for another 30-45 minutes, or until the potatoes are tender.

In a small bowl, whisk together the sour cream and all-purpose flour until smooth. Take a spoonful of the hot goulash liquid and whisk it into the sour cream mixture.

Slowly stir the sour cream mixture into the goulash. Season with salt and freshly ground black pepper to taste.

Simmer the goulash for another 5-10 minutes, stirring occasionally, until the sauce has thickened slightly. Do not boil after adding the sour cream.

Serve hot, garnished with fresh chopped parsley.