Simmer the crushed tomato for about 10 minutes. Add the white tuna belly and set aside.
To make the béchamel sauce, melt the butter in a saucepan, add the flour and mix well. Gradually pour in the warm or hot milk. Stir well over a low heat until it thickens, season to taste and remove. Slice the aubergine and grill it on the griddle.
To finish the dish, alternate different layers of aubergine and the tuna and tomato sauce. Top with the béchamel sauce and garnish with basil.