Preheat oven to 450°F.
Line a 9- to 10-inch tart pan with purchased piecrust. Trim edge of pastry even with edge of tart pan. Do not prick piecrust. Line the piecrust with a double thickness of heavy foil. Bake 8 minutes; remove foil. Bake 4 to 6 minutes more or until piecrust is set. Remove from oven. Cool piecrust while preparing filling. Reduce oven temperature to 400°F.
In an extra-large skillet, melt butter over medium-high heat. Add Brussels sprouts. Cook 5 minutes or just until tender. Add onion and squash. Cook and stir for 5 minutes or until onion and squash are just tender. Add yellow pepper and salt. Cook 3 to 5 minutes more, stirring frequently, until vegetables begin to brown. Remove from heat.
Spread baked tart shell with semisoft cheese. Spoon cooked vegetables atop cheese. Top with tomatoes and black pepper. Bake for 15 minutes or until tomatoes are softened. Cool for 30 minutes. Cut into wedges to serve.