We salted the tomato slices and let them stand for 30 minutes to draw out any excess moisture that could make the pie soggy.
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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