In a 6-quart saucepan, melt the butter over medium-high heat.
Add the shallots and garlic; cook, stirring occasionally, until soft, about 4 minutes.
Add the Brussels sprouts and stock, season with salt and pepper, and bring to a boil; cover and cook until wilted and soft, about 5 minutes.
Uncover, stir in the cream and sugar, and cook until the liquid is reduced by half, about 3 minutes more.
Season with salt and pepper; serve.
