Rub the pepper halves with 1 tbsp olive oil and put under a hot grill, skin-side up for 10 minutes until charred. Leave to cool then peel off the skin and remove the stalk and seeds. Whizz in a blender, adding a few tbsps of the passata if needed. Fry the garlic in 1 tsp olive oil, before adding the passata, red pepper, vinegar and seasoning. Simmer for 10 minutes. Boil the potatoes for about 15 minutes until tender. Season and then brush the tuna steaks with 1 tsp olive oil. Griddle the tuna for 1-2 minutes on each side. Serve with potatoes, sauce and a few parsley leaves.