6 12cm pieces marrow bone, halved lengthways (see note)2½ tbspsea salt flakes6 5cm-thick slices white breadFor buttering: softened butterCaramelised onion2 tbsp grapeseed oil or olive oil2white onions, finely dicedChimichurri3garlic cloves½ jalapeño chilli, coarsely chopped, half the seeds reserved½ cup(firmly packed) flat-leaf parsley, finely chopped80 ml (1/3 cup) extra-virgin olive oil70 ml white vinegar2 tbsp (firmly packed) oregano, finely chopped2 tsp dried red chilli flakes
Place marrow bones cut-side up in a single layer in an oven tray, scatter salt over, cover and refrigerate for 20 hours.
Meanwhile, for caramelised onion, heat a large frying pan over medium-high heat. Add oil then onion, stir occasionally until onion starts to become translucent (4-5 minutes), reduce heat to low-medium, stir occasionally until deeply caramelised (25-30 minutes), season to taste and keep warm.
Meanwhile, for chimichurri, pound garlic and jalapeño in a mortar and pestle to a smooth paste, transfer to a bowl, combine with remaining ingredients, season to taste with sea salt and set aside.
Preheat oven to 220C. Rinse marrow bones under cold running water, pat dry with absorbent paper, place cut-side up on an oven tray and roast until cooked through (4-6 minutes). Butter both sides of bread slices. Heat a large frying pan over medium heat, add bread and cook, turning once, until toasted (1-2 minutes each side), then halve diagonally and keep warm. Transfer to plates with marrow bones, top with chimichurri and caramelised onion and serve hot.
Note Ask your butcher to halve the marrow bones