Combine mayonnaise, shallots and basil in a small bowl and set aside.
Preheat the oven to 400 degrees F. Place the pancetta in a single layer on a parchment lined jelly roll pan. Bake for 8 minutes or until crisp. Drain on paper towels.
Toast the bread slices. Spread mayonnaise mixture evenly over the bread slices. Top each of the 4 bread slices with 1 pancetta slice, 2 tomato slices and a few leaves of arugula. Top sandwiches with remaining 4 bread slices. Serve.