In a small saucepan, simmer brown sugar and water, stirring, until the sugar dissolves, about 5 minutes; let cool. In a cocktail shaker, crush pear, cloves and allspice berries with a muddler or wooden spoon. Fill the shaker with ice, then add brown sugar syrup and lemon juice; shake well. Strain into 2 flutes. Top with champagne and garnish each with a pear slice.