This was really tasty, and stupidly simple. It's slightly modified from Deborah Madison's Vegetarian Cooking For Everyone.
Chop 1 large onion into big chunks.
Sautee in olive oil with some rosemary (either fresh or dried,) until soft and browned slightly. Season with salt, pepper, and red pepper flakes (or a good sprinkle of cayenne,)
Add:
1 lb. white mushrooms thickly sliced (one could also quarter these, I think)
1 lb. portobello mushrooms cut into large chunks (the recipe calls for 1/2 lb, but I love portobellos, so I doubled it.)
Sautee until everything is getting soft, and the mushrooms are giving off their juice.
Add:
2 tbsps tomato paste
1.5 cups liquid (combination of white wine, mushroom stock, veggie stock, water... whatever you have,)
2 tbsps sherry vinegar (or red wine vinegar, or some other vinegar... cheapish balsamic might work well,)
Stir until the tomato paste has dissolved, and reduce heat to a simmer. Let simmer for 15-30 minutes, until the liquid has thickened, and everything is all dark and oozy and wonderful.
The recipe says to swirl with butter at the very end, and to use 1/4 cup of olive oil in total (you're supposed to sautee the onions, then remove, then sautee the white mushrooms separately, then sautee the portobellos separately, but I didn't bother. And I only used less than a tbsp of olive oil, and it was fine.) I didn't bother with the last step.
This was good on its own, but would be better served over mashed potatoes, or polenta, or with a good crusty bread to soak up the juices.