Preheat the oven to 400 degrees F.To make the batter, combine the flour, beef bouillon and mustard powder in a mixing bowl. Add the eggs and milk and whisk continuously for several minutes until the batter is perfectly smooth and airy. Let sit at room temperature for at least 30 minutes before using.
While the batter is resting, rub the sausages with olive oil. Grease a medium sized casserole dish (about 8x10 inch) or 12 inch cast iron skillet and place the sausages in it. Roast the sausages for about 10 minutes on each side until browned.
Carefully remove the casserole dish and increase the oven temperature to 450 degrees F. The fat from the sausages is traditionally left in to contribute flavor and texture to the batter.While the casserole dish is still very hot, pour the batter over the sausages. Sprinkle with the fresh herbs. Bake for 15-20 minutes or until the Yorkshire pudding has risen and is browned. Reduce the temperature to 350 degrees F and continue to bake for another 10 minutes or until the batter is cooked through. (DO NOT OPEN THE OVEN during the first 20 minutes or the batter will not rise properly).Serve immediately with homemade onion gravy and your choice of vegetables.