Preheat oven to 375°F.
Dice the potatoes and place in a medium size baking dish.
In a small saucepan, over medium-high heat, whisk together the butter and flour until smooth.
Gradually add 1 cup of the heavy cream and let come to a boil, stirring constantly.
Once the sauce begins to thicken, add the remaining 1/2 cup of heavy cream.
Lower the heat and keep stirring until thick and smooth, about 2-3 minutes.
Whisk in the garlic, lemon zest, rosemary, salt, and pepper.
Remove the pan from the heat and whisk in the lemon juice. Taste test the sauce and add more lemon juice, salt, and/or pepper, as desired.
Pour the cream mixture over the potatoes and cover with foil.
Bake for 60 minutes or until the potatoes are fork-tender.
Top with fresh rosemary sprigs, if desired, and serve hot.