Bring a large pot of salted water to the boil, and cook the new potatoes until they are tender. Cut into bite size pieces.
In a separate bowl, combine the sour cream, 2 to 3 tablespoons of the tomato and basil sauce and minced garlic, then stir in the potatoes and spring onions, stirring well to ensure everything is coated.
Season to taste with salt and pepper.
Meanwhile, heat a little olive oil in a small, heavy based frying pan over a medium to high heat.
Season the tuna steak well with salt and pepper and place it carefully in the pan.
Cook for 1 to 2 minutes each side, and then set aside to rest for a minute or so.
In the pan you have cooked the fish in, add the wine and reduce down to a couple of tablespoons.
Add the rest of the tomato and basil sauce and the olives and heat through.
Serve the fish with the salad, pouring the sauce over the fish.