1. Preheat oven to 450 degree F and arrange rack in middle of oven. Gently press piecrust into a 9-inch round fluted tart pan with removable bottom. Prick bottom of crust with a fork and chill until firm, about 15 minutes. Bake until golden, about 12 minutes. Transfer until golden, about 12 minutes. Transfer tart shell in pan to a rack to cool.
2. Meanwhile, halve, pit, and scoop out flesh from avocados. Cut into 1/4-inch-thick slices and transfer to a large plate. Drizzle with juice; salt and pepper to taste.
3. Toss together lettuce, scallion, mayonnaise, bacon, and chopped basil in a large bowl; salt and pepper to taste.
4. Remove sides of pan and place tart shell on a serving platter. Arrange avocado slices in a single layer on bottom of shell. Top with lettuce mixture and tomato, overlapping slices. Garnish with basil leaves. Cut into wedges and serve. Makes 8 servings.