Preheat the oven to 450°F. Put the oil and butter into a 9-10-inch ovenproof skillet and put it inside as it heats.
In a medium bowl, whisk together the eggs, flour, milk and salt.
Carefully pour the batter into the hot skillet and return it to the oven. Bake for 15 minutes, or until puffed and golden.
To make the hollandaise, put the egg yolks, mustard, lemon juice and salt into a blender; cover and pulse until blended.
Melt the butter on the stovetop or in the microwave until melted and hot; with the blender on high speed, slowly pour the butter through the hole in the top.
Blend until it thickens to a creamy consistency.
Thinly slice the roast beef and serve it in the Dutch baby, drizzled with hollandaise or gravy.