Add potatoes to a large pot and cover with cold water. Stir in 1 tablespoon of salt. Bring to a boil, then reduce to a simmer. Cook for 15–20 minutes or until fork-tender.

While potatoes cook, heat butter and cream in a saucepan over medium-low heat. Add smashed garlic and rosemary. Simmer gently for 10 minutes. Discard rosemary. Mash garlic into the cream if a stronger flavor is desired.

Drain potatoes well and return to the pot. Mash using a potato masher or ricer until smooth.

Stir in the warm infused cream a little at a time until fully incorporated. Add remaining ½ teaspoon salt and black pepper. Stir until creamy and well mixed.

Transfer to a bowl, garnish with a rosemary sprig or a small pat of butter, and serve immediately.