Heat olive oil in a large pot over medium-high heat. Season the oxtail pieces with salt and black pepper. Once the oil is hot, brown the oxtail pieces on all sides.

Remove the browned oxtail and set aside. In the same pot, add the chopped onion, leeks, green bell pepper, carrots, and garlic. Sauté until the vegetables are soft and lightly browned.

Add the red wine and sweet red vermouth to the pot. Cook for a few minutes to let the alcohol evaporate.

Transfer the oxtail pieces to a pressure cooker. Add a few whole black peppercorns and the brandy, stirring to combine.

Add the sautéed vegetables and their liquid to the pressure cooker. Pour in enough water to cover everything. If desired, add a beef bouillon cube.

Seal the pressure cooker. Once it reaches pressure, cook for 30 minutes, then remove from heat.

Let the pressure release naturally. Remove the oxtail pieces and set aside. Blend the cooked vegetables and some of the cooking liquid until smooth.

If the sauce is too thick, add a bit more of the reserved cooking liquid until you reach your desired consistency.

Return the oxtail pieces to a clean pot, pour the sauce over them, and heat everything together until hot. Serve warm.