Heat grill to high. Brush the tuna steaks with the 1 tablespoon of oil and season and salt and pepper. Grill for 3 to 4 minutes on both sides or until just cooked through. Let rest for 5 minutes then flake with a fork. Transfer to a bowl with the red onion and tomatoes.
Whisk together the anchovy paste, lemon juice, mustard, red pepper, and 1/2 cup olive oil in a small bowl. Add the capers and then season with salt, to taste, if necessary. Pour the dressing over the tuna, add the basil, and gently mix to combine.
Place the baguette on the grill, close the cover and let heat through, about 3 to 4 minutes. Remove from the grill and slice lengthwise. Remove some of the inside of the bread. Spread the tuna mixture evenly over the bottom half of the bread, place the top bread on, and press down firmly on the sandwich. Wrap the sandwich in foil and place a heavy pan on top. Let stand 30 minutes before serving and then cut into 4 pieces.