Step 2: Soften the Onion in Butter - Melt the butter in a
large skillet over medium heat, then add the diced onion. Let it cook for about 8 to 10 minutes, stirring now and then, until the onion softens and turns lightly golden. This step gives the casserole that gentle sweetness you expect from a good French dip. Do not rush it too much. The onion is small, but it does a lot of the heavy lifting for flavor.