Prepare your tomatoes. Lightly cut out an X shape on the bottom of the skins. Place in a bowl and pour boiling water over the top. After 1-2 minutes, take them out, drain the water, and place them back in the bowl.
4 large yellow tomatoes
Peel the tomatoes (the skins should come away easily after being soaked) and chop them into 3-4 large pieces each, slicing from top to bottom, discarding the stem and the toughest part of the core. With the pieces that remain, scoop out the big clusters of seeds and membrane, but there is no need to be a perfectionist about it. We don't want to drain too many of the juices, and a few seeds are OK. Place the tomatoes in a bowl and set aside.
Heat 1 Tbsp of the olive oil in a wide bottomed, casserole dish or saucepan. Over a very gentle heat, saute the onions and garlic until they are soft. Try not to let anything go brown in the process.
1/2 white or brown onion, 4 cloves garlic, 4 Tbsp olive oil
Bump up the heat and add about 1/3rd of the tomatoes to the pan, and mix through, allowing them to start cooking and releasing juices. Add another 1/3rd to the pan, and once they're starting to break down, add the remaining 1/3rd. Mix constantly, breaking up the tomatoes with the spoon.
You can use a potato masher to encourage the tomatoes to break up further, if you wish.
Add the remaining 3 Tbsps of olive oil, balsamic vinegar and sugar to the pan. Bring to a gentle simmer, cover and allow to cook for 1-2 hours, until a thick sauce has formed and the oil starts to come away from the tomatoes.
1/4 tsp balsamic vinegar, 1 tsp sugar
Finally, mix in the mascarpone and parmesan until melted through.
1 Tbsp mascarpone, 1 handful parmesan
Add the basil leaves, stir through.
1 bunch basil
Serve.