Preheat the oven to 190C/375F. Generously grease 6 holes of a muffin tray.
Use a mandoline to thinly cut them into 2mm slices (discard the very ends of the potatoes). I recommend using a finger guard, at least when you get closer to the blade. Use kitchen roll to dry the slices as best you can.
Toss the slices in a large bowl with olive oil, rosemary, garlic, salt and pepper until everything is well-distributed. Place 1/6th of the slices in a row, overlapping by around 2/3-3/4 of the previous slice (usually around 15 slices). Sprinkle 1/6th of the parmesan over the slices (around 1 tbsp) then place a slice of bacon next to them.
Roll up the potato slices then wrap them in the bacon. You might find it a little fiddly at first, but you'll get the swing of it. They don't have to look perfect, so long as they're properly rolled up. Place in the muffin tray and gently fan out the petals so it looks like a rose. Repeat 5 more times to create 6 roses. Try and work quickly as the longer the slices sit the more watery they get. Not a huge issue as the moisture will evaporate in the oven, they're just more difficult to work with.
Place the roses in the oven uncovered for 20 minutes. They won't be cooked through at this point, but will start lightly browning on the edges. Take the tray out and tightly cover the roses with foil, then place back in the oven for 35-45 minutes, or until the potatoes are crisp and tender right the way through.
Use a spoon to carefully remove the roses from the tin then tuck in and enjoy!