Steam green beans in a large pot lined with a steamer basket and filled with water to the bottom of the basket (alternately, you can boil the beans in a large pot filled with about 3 quarts of salted water). In both cases, you'll want to cook just until crisp-tender: bring water to a boil, add beans, and cover and cook on medium for about 5 minutes, depending on size.
Meanwhile, melt 1 tablespoon butter in a skillet over medium-low heat. Add chopped onions and cook, stirring occasionally, until caramelized, 5-8 minutes. Add garlic variation here if using.
Add balsamic vinegar and cook a minute before adding the tomatoes, salt, pepper, and dried chili peppers and any optional herbs. Cook another 1-3 minutes, and then remove from heat.
Drain the beans when they are done and toss with the remaining butter and more salt and pepper as you'd like.
Serving Options: 1)Place the beans on a serving plate and top with the onion-tomato mixture, OR 2) Toss it all together and serve in a bowl. Top with cheese variation if desired.
*My preference is drier paste tomatoes, but you can also use halved grape or cherry tomatoes, or chopped larger heirloom tomatoes (though squeeze juice and seeds from the halves before chopping).**Or olive oil for a dairy free version.Optional Variations:
Add fresh herbs (2 tablespoons thyme and rosemary, or 1/4 cup chopped basil) at the same time as the tomatoes.
Add 1-2 cloves of garlic, minced, at the end of caramelizing the onions.
Top the finished dish with 1/4 cup crumbled or shredded cheese like feta, goat, or parmesan.