Preheat oven to 350°F (180°C). Line 8-in.(20-cm) square baking pan with parchment paper, allowing for overhang on sides. Set aside. Cook bacon in non-stick skillet over medium heat until golden brown and crisp, about 8 min. Drain on paper towels.
In bowl, whisk eggs with milk, chives, salt and pepper. Stir in bacon. Pour frittata mixture into lined baking pan. Bake 20 min. until egg mixture is set. Cool slightly; cut into 12 pieces.
Meanwhile, mix mustard with maple syrup. Split baguette in half lengthwise; cut into 12 sections. Fill each with piece of frittata, strip of cheese, arugula and drizzle of maple mustard.
Substitute Dijon mustard with grainy mustard.