Stir together lemon juice and garlic in a small saucepan; cook over medium heat 1 to 2 minutes. Stir in basil and next 3 ingredients; bring to a boil. Remove from heat.
Brush mushrooms with herb mixture, applying generously to the underside.
Divide mascarpone into 8 portions, and roll into balls.
Grill mushrooms, top side up, covered with grill lid, over high heat (400º to 500º) 5 to 7 minutes. Turn mushrooms, and top each with a cheese ball. Grill, covered with grill lid, 5 minutes. Garnish with basil sprig, if desired. Serve immediately.