Place tomato slices on paper towels, and sprinkle with 1/2 teaspoon salt; let stand 30 minutes.
Unfold piecrust, and fit into a 9-inch deep-dish pieplate. Fold edges of piecrust under, and crimp. Line with aluminum foil, and fill with pie weights or dried beans.
Bake at 425° for 10 to 12 minutes or until lightly browned. Remove pie weights and foil.
Cook bacon in a large skillet until crisp; remove bacon, and drain on paper towels, reserving 1 teaspoon drippings in skillet. Crumble bacon, and set aside.
Sauté onion in hot drippings in skillet over medium-high heat 8 to 10 minutes or until tender. Spoon onion over prepared piecrust, and top with tomato slices.
Stir together Cheddar cheese, mayonnaise, pepper, and remaining 1/2 teaspoon salt. Spread mixture over tomato slices. Combine Parmesan cheese and breadcrumbs; sprinkle over top.
Bake at 350° for 30 minutes or until golden. Let stand 5 minutes before serving. Sprinkle with bacon.
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