Preheat the oven to 325°F. Add the ground beef, red onion, bread crumbs, egg, mustard, paprika, and garlic to a bowl, season with salt and pepper, and mix well.
Remove the leaves and stalk from the head of cauliflower and stuff the inside with the ground beef mixture. Place the head of cauliflower, meat side down, in a tart pan and cook for 60 minutes.
Meanwhile, peel the potatoes and boil them in salted water. Add the cooked potatoes to a bowl, together with the butter, egg, and cheese, and mash until smooth. Fold in the cooked bacon bits and spoon the cheesy bacon mashed potatoes into a piping bag.
Combine the tomato paste, egg, and paprika, and brush the mixture over the cooked cauliflower.
Pipe the cheesy bacon mashed potatoes in rings around the stuffed cauliflower.
Use a spoon to create “sun rays” from the stuffed cauliflower to the edge of the pan.
Put the pan back in the oven and cook for another 15 minutes.