saŭco

One root, not two. Saŭc- is indivisible. There is no suffix -c- in Esperanto, and the aŭ in the middle is a diphthong belonging to the root, not the conjunction aŭ (or) that you already know. The word is two syllables, saŭ-co, and the temptation to hear it as three is the main thing to get right.

The genuine relation is worth knowing. Salo is salt, and it comes from Latin sal. Saŭco comes from Latin salsa, which is what sal does when it becomes an adjective meaning salted. So sauce, salsa, salad and sausage are all, historically, salted things, and Esperanto happens to carry two members of that family as separate roots. This is the same sort of gift as kvar and kvadrato further down this band: the etymology helps you remember the word without giving you any rule you can apply.

Cooking with it. Saŭcujo is the sauceboat, using -uj-, the container suffix. Saŭci is to sauce something, saŭcita sauced. The everyday collocations behave as you would expect: tomata saŭco, blanka saŭco, saŭco por la fiŝo. Note that Esperanto uses saŭco for gravy too, so viandsaŭco or rostaĵa saŭco is what you reach for when the distinction matters.

Not to be confused. Supo is soup, a dish in its own right; saŭco is the liquid that accompanies another dish. Kaĉo is porridge, and figuratively a mess. A sauce that has gone wrong is fuŝita saŭco and, if you are being funny about it, kaĉo.