Roast potatoes with gremolata (peeler)

Potatoes

• 1200 g floury potatoes, red and blue varieties
• 2 tsp olive oil, plus 3 tbsp to roast
• 1200 g water, plus extra to parboil
• 1 ½ tsp sea salt, plus extra to season
• 3 sprigs fresh rosemary

Parmesan gremolata

• 90 g Parmesan cheese, cut into pieces (3 cm)
• 1 ½ garlic cloves
• 3 sprigs fresh flat-leaf parsley, leaves only
• 1 ½ lemons, peel only, no white pith