This bistro classic is deceptively easy to make. It just takes a little time and a few good ingredients. We found that yellow onions, also known as Spanish onions, work best for this recipe. They have a nice balance of sweetness and bitterness and caramelize perfectly. Just don't take them too dark -- a nice golden brown is perfect. Good beef broth also makes a big difference. If you can't make your own, look for a high-quality prepared variety.