How to Make It
Drain the tomatoes in a strainer set over a bowl. Scoop out and discard any seeds from the tomatoes. Crush the tomatoes well and place them in a separate bowl; reserve their liquid for another use.
In a large nonreactive saucepan, cook the onion in 6 tablespoons of the butter over moderately low heat until golden. Add the tomatoes and salt and simmer over moderate heat for 20 minutes.
Just before serving, stir in the remaining 2 tablespoons butter and the basil and season with pepper.