Preparation Place 10 berries in freezer; reserve for garnish. Purée remaining berries in processor. Strain through sieve over bowl, pressing on solids. Mix in sugar. (Purée can be made 1 day ahead. Cover and refrigerate.) Whisk orange juice and concentrate in pitcher to blend. Mix in champagne. Divide mimosa among 10 champagne glasses. Drizzle 1 1/2 teaspoons berry purée over each. Garnish with orange slices and reserved berries.
Preparation Place 10 berries in freezer; reserve for garnish. Purée remaining berries in processor. Strain through sieve over bowl, pressing on solids. Mix in sugar. (Purée can be made 1 day ahead. Cover and refrigerate.) Whisk orange juice and concentrate in pitcher to blend. Mix in champagne. Divide mimosa among 10 champagne glasses. Drizzle 1 1/2 teaspoons berry purée over each. Garnish with orange slices and reserved berries.