In a medium bowl, whisk together the flour, eggs, milk, mustard and fresh herbs. Cover and let stand 1 hour.
Preheat oven to 425 F. Place oil in oven-proof skillet or baking dish (enough for a thin layer over the entire bottom) and place in the preheated oven while you prepare the sausages.
Wrap each of the sausages with a strip of bacon. Remove hot skillet with oil from the oven. Place the sausages in the middle and scatter the red onion slices around the outside. Place in the oven for 15 minutes, then remove, flip sausages and stir onions. Return to oven for a further 15 minutes. Have a baking sheet handy and re-whisk the batter near the end of this cooking time.
Remove from oven. Place skillet on the baking sheet and quickly pour the batter over-top. Return to oven and allow to cook, without opening the oven door, for 25-30 minutes, or until puffed and golden.