You can well imagine this might be made from tomatoes. Correct.
The process of thickening this time however is Reduction: the process of thickening or intensifying flavour by evaporating liquid via cooking.
Sauté onions in oil, preferably olive.
Bring heat up so they come to a slow boil
Simmer until thickened to desired consistency
Try on top of pasta or rice
Add more flavourings such as olives, hot pepper flakes, capers, or vegetables such as peppers, zucchini, and eggplant and of course herbs such as oregano, thyme and rosemary.