Slice the tomatos in half lengthwise. Very lightly brush the tomatos with olive oil and season with salt and pepper.
Roast for 30 minutes, flipping once, until tender and edges are a bit charred. Not too much though. You don't want that taste to come through in the final product. Allow to cool slightly before slicing or dicing.
Meanwhile, peel and slice the onion. In a large saute pan, add the margarine and olive oil and saute very slowly until golden and sweet. This takes awhile. Even up to an hour. You can always expedite the process by adding a teaspoon of sugar which helps in the the caramelization process. This is completely optional.
Add the sliced tomatos to the onions along with the balsamic and thyme leaves. Saute until thickened and it reaches a jam-like consistency.