Potato roulade filled with mushrooms

Soft and flavourful, with fillings of choice: spinach, radicchio and Gorgonzola, mushrooms and ham. Convenient made ahead and baked when needed.
INGREDIENTS FOR 4/6 PEOPLE
1 kg potatoes
200 g flour
1 egg
Salt, pepper
FILLING
500 g button mushrooms
1 garlic clove
Parsley
Coarsely grated Parmigiano or Grana Padano
I clean mushrooms and cook on high with oil and garlic, then add salt and pepper.
After about fifteen minutes, I remove garlic, add parsley and cool.
I microwave or boil potatoes.
I peel and rice hot onto the worktop.
I make a well, add egg beaten with salt and pepper and gradually flour. I used less than 200 g; keep soft, not sticky.
I quickly form a ball.
I oil parchment and flatten dough by hand into a 35 x 28 rectangle, then add mushrooms.
I add plenty of coarsely grated Parmigiano.
I roll with parchment into an even log, seal ends and wrap in parchment.
I wrap in a clean towel, tie ends like a sweet and simmer in a long pan of salted water for 30 minutes.
I remove and cool, generously butter a disposable dish for easy cleanup. The cold roll firms and slices well.
I cut about 2 cm slices, wiping blade between cuts.
I arrange without overlap and add Parmigiano and melted butter.
Cover and refrigerate until needed, or bake at 180°C for about 15/20 minutes until golden.
A variation is sautéed spinach, cooked ham and mild Provola.