Heat oven to 190C/fan 170C/gas 5. Split the pork fillets lengthwise almost in half and open out like a book. Bash with a rolling pin to flatten, then sprinkle on all sides with salt, pepper and lemon juice. Fill the pork with the black pudding, folding the meat back over it to enclose it.
Stretch the bacon rashers with the back of a knife, then wrap around the pork fillets, tucking the ends under the pork where possible. Transfer to a large roasting tin, drizzle with the oil, then roast for 30 mins.
Meanwhile, heat the honey in a pan, then toss the rhubarb in the honey. Add to the roasting tin, then return to the oven for 10-12 more mins until the rhubarb is tender and the bacon nicely browned. Transfer the pork and rhubarb to a warm plate and keep warm while you make the sauce.
Set the tin on the hob and add the stock. Bring to the boil, stirring to scrape all the pan juices from the base of the tin. Bubble for a few mins, then stir in the crème fraîche and whisk until it has dissolved into the sauce. Taste and adjust the seasoning if necessary.
Cut the pork into slices and serve with the rhubarb and a little sauce poured over. Serve the remaining sauce separately.