In a small sauté pan, melt the remaining 2 tablespoons butter over medium heat and cook until the milk solids sink to the bottom of the pan and turn hazelnut
brown and the butter starts to smell nutty, about 3 to 4 minutes. Add the rosemary and cook, stirring, until fragrant, about 1 minute. Add the scallions and cook, stirring, 1 minute. Transfer the rosemary-scallion butter to the bowl with the mashed potatoes and add the crème fraîche, 3/4 cup Gruyère, lobster meat, and lobster butter. Mix until thoroughly combined. Add 1 tablespoon heavy cream and mix until incorporated. Add more heavy cream if needed to achieve the desired consistency, and season to taste with salt and pepper.