Potato Parsnip Mash

Bahan-bahan (6 porsi)

• 1 lbs russet (memanggang kentang, kupas dan potong 2 inci)
• 1 lb parsnip (kupas dan potong 1 inci)
• 3 cloves bawang putih
• 1 setangkai rosemary
• 1/2 tbsp salt (plus more to taste)
• 1/4 cup light sour cream
• 1/4 cup 2% milk
• 1 tbsp mentega tawar
• pinch of black pepper