Pasta with Balsamic Onions

Boil salted water for pasta and cook it; meanwhile, in a tablespoon or so of olive oil, sear a couple of sliced onions until nicely browned, stirring almost all the time. Splash some balsamic vinegar over all and sprinkle with salt and lots of black pepper; reduce the heat so the mixture thickens into a sauce. Drain the pasta, reserving some of the cooking water; toss the pasta with the onion sauce, adding some of the reserved cooking water as needed to moisten and serve; Parmesan cheese is optional.

— Recipe from Mark Bittman’s Kitchen Express