Start preheating the oven to 350°F (175°C).
Preheat a tablespoon of olive oil (or butter if you prefer a richer flavor) in a large, heavy-bottomed frying pan (such as copper, cast iron, or carbon steel) over medium-high heat. Sear the meat on one side for about 1 - 1.5 minutes, undisturbed, until nicely browned. Then flip and sear for an additional 1-1.5 minutes on the other side. Transfer the meat to a 5-quart (5L) cast iron pot. Season with 1/2 tsp of salt and a pinch of pepper in the pot.
Add another tablespoon of olive oil (or butter) to the same pan. Add the chopped onions and mushrooms, then season with 1/2 teaspoon of salt and a pinch of pepper. Cook over medium-high heat until lightly browned, stirring occasionally. Transfer them to the cast-iron pot on top of the meat, followed by two-thirds of the chopped greens and potatoes layered on top.
Deglaze the frying pan with 1 1/2 cups of hot water and 1/4 cup of white wine. Keep the heat at medium-high. Once the liquid is simmering, add a cube of beef bouillon and crush it to dissolve. Taste for salt and adjust if needed. It should be salted perfectly or just slightly salty. Pour the liquid into the cast iron pot. Add two tablespoons of butter on top.
Cover the pot with a lid and transfer it to the preheated oven. Cook for 2 hours and 10 minutes. Next, carefully open the pot and add four heaping tablespoons of sour cream and two more tablespoons of butter on top. Cover and cook for another 20 minutes. Remove from the oven and stir well but gently. Serve immediately, garnished with chopped fresh greens, and along with fresh vegetables, a salad, or pickled vegetables (see links to my recipes in the post abov).