Preheat the oven to 190°C/fan 170°C/gas mark 5. Prick the potatoes and place in the preheated oven for 45 minutes. Brush the peppers with olive oil and cook under the grill for about 5 minutes, turning them over, until slightly blackened. Transfer the grilled peppers to a bowl, cover the bowl tightly with cling film and leave until cold.
For the burgers, remove the bone marrow from the bone, cut each piece in half and place in the fridge until you are ready to use it. Sauté the onion over a medium heat in 2 tablespoons of the vegetable oil for a few minutes until softened but not coloured. Stir in the marjoram and leave to cool off the heat.
Put the steak on a chopping board. Trim off and discard any excess fat and sinew, then chop the meat with a very sharp knife. Do the same with the back fat. Place the meat and fat in a food processor and pulse for a few seconds to mince even further.
Put the minced meat and fat in a bowl with the cooled onion and season with salt and pepper. Mix well, then shape into 4 patties, each about 2cm thick. Cover and keep in the fridge until needed.
For the salsa, first peel the skins off the peppers and chop the flesh. Mix the chopped peppers in a bowl with the red onion, tomatoes, garlic and chilli.
Add the extra virgin olive oil, vinegar, parsley and some salt and pepper, mix well, then taste and adjust the flavourings as you like. Keep in a covered bowl in the fridge until ready to serve.
Prepare the chunky chips by cutting the pre-baked potatoes into wedges and shallow-fry in hot vegetable oil over a medium to high heat for 4–5 minutes, until golden brown on their cut sides.
Heat 2 tablespoons of vegetable oil in a heavy-based frying pan until hot. Fry the burgers over a medium heat for 3 minutes on each side if you like them medium-rare, longer if you prefer them well-cooked. Remove and set aside to rest for a couple of minutes.
Dust the pieces of bone marrow with the crushed peppercorns. Heat the 2 tablespoons of olive oil in the pan used to cook the burgers and fry the bone marrow for a minute on each side, until caramelized. Place a piece of marrow on top of, or next to, each burger and serve with the salsa and chips.