Heat the grill or set the oven to the highest temperature.
Place the pepper slices under the grill or in the oven, cut side down, until the skin darkens and blisters. Take out and cool, then skin.
Lay the tuna steaks on an oiled grill rack and grill for 2-3 minutes on each side. Season with salt and pepper, drizzle a little lemon juice over the top and leave to cool slightly.
Put the garlic in a bowl with the mustard, vinegar, olive oil and mayonnaise, season with salt and pepper and mix to a make a dressing.
Halve the rolls and drizzle a little dressing over the top.
Arrange the peppers and radicchio leaves on the bottom halves of the rolls and place the tuna on top. Drizzle with dressing and cover with the top halves of the rolls.