Combine the butter, flour, salt, and egg and knead into a smooth dough. Wrap the dough in plastic wrap and refrigerate for 30 minutes. Roll out the shortcrust pastry dough and place it in a 10-inch tart pan. Cover the dough with parchment paper, fill with baking beans or pie weights, and blind bake the crust in a preheated oven at 350°F for 20 minutes.
Whisk the cream cheese and milk together until smooth and liquidy, combine with the corn starch and salt, and then pour the cream sauce filling over the cooled shortcrust pastry crust.
Lay the bacon slices out in a row so that the ends slightly overlap and top with the potato slices, red onion slices, and cheddar cheese slices. Roll everything up into a giant roll and freeze for 15 minutes.
Place the onion rose in the cream sauce filling in the tart pan.
Heat the honey in a saucepan over medium heat, dip the rosemary sprigs in the honey, and brush the sprigs over the onion rose tart. Transfer the tart pan back to the oven and cook at 350°F for 45 minutes.
