This is a perfect side dish or starter and uses two of the frozen veggies you'll often see in your Winter Shares-- green beans and tomatoes. It also easily doubles for a larger portion-- use a full quart of green beans and a full pint of frozen tomatoes, doubling the rest of the ingredients too.
Heat the oil over medium heat. Add in the onion, and cook for about 2 minutes. Toss in the mushrooms and stir, and allow them to release their liquid and cook down, about 5-7 more minutes. Add in the garlic, a pinch of salt, and the rosemary and cook for an additional minute or two all together, being careful not to burn.
Add in the tomatoes and the balsamic, and stir well. Allow to bubble, and make sure nothing is sticking to the bottom of your pan.
Add in the green beans. If using thawed, once-frozen beans, only cook long enough to just heat them through (only a about 2 minutes) so you don't over-cook and make them mushy. If using fresh beans, cook till almost softened, 5-8 minutes.
Serve warm, with a few shavings of your cheese on top (optional.)