1. Preheat oven to 425°. Melt butter in a large skillet over medium-high heat; add onion and, if desired, fennel, and cook, stirring often, 20 minutes or until golden brown.
2. Flour hands, and shape each Pizzetta Dough ball into a 6- to 8-inch round. Place each round on a small piece of parchment paper. Brush with 1 Tbsp. oil. Top dough rounds with cheese, next 2 ingredients, and onion mixture.
3. Place 4 pizzettas (on parchment paper) directly on oven rack, and bake at 425° for 12 to 14 minutes or until golden. Repeat with remaining dough rounds.
4. Toss together arugula, next 2 ingredients, and remaining 1 Tbsp. oil. Add salt and pepper to taste. Sprinkle baked pizzettas with arugula mixture just before serving.
*1 lb. store-bought pizza dough and 8 oz. sliced or shredded mozzarella cheese may be substituted.