Add the butter and olive oil to a large skillet. When the butter has melted, add the onions and sauté over a low-medium heat for 20-25 minutes, stirring occasionally.

Set half of the onions aside, then transfer the rest to a blender.

Add the beef broth and Dijon mustard, then blend until smooth.

Return the blended mixture to the skillet, and bring to the boil. Reduce to a simmer, and add some extra butter for smoothness.

Test for seasoning, and add salt and pepper as necessary. Add the reserved onions back into the skillet, stir well, then transfer the gravy to a serving vessel.