Step 1 Spray a grill pan with cooking oil and set over medium–high heat. Cook capsicum, skin-side down, for 8–10 minutes, or until skin is black and blistered. Place capsicum in a zip-lock bag and seal; set aside for 15 minutes.
Step 2 Meanwhile, place remaining ingredients, except sourdough, in a large bowl; toss lightly to combine. Set tuna mixture aside for 10 minutes.
Step 3 Set the same grill pan over medium–high heat. Add slices of sourdough and cook, turning, for 5 minutes, or until toasted.
Step 4 Peel or rub skin from reserved capsicum; discard skin. Slice into strips and add to reserved tuna mixture; toss to combine.
Step 5 Divide toast among plates, top with tuna mixture and extra basil leaves, and serve.