In a large bowl with your hands, lightly combine flour, salt, and butter until it resembles crumbles. Mix in water a bit at a time. When it begins to stick together, form dough into a round disc. Place in a sealed container and refrigerate 30 minutes.
Preheat oven to 350 F. Roll dough out on a floured surface until it's larger than the 9-inch tart pan. Transfer dough to the pan, pressing into the corners and leaving 2 to 3 inches of dough overlapping the sides. With a fork, pierce dough on bottom of pan many times. Fit parchment paper into tart crust and fill with pie weights (dry beans or rice.) Place on a baking sheet and bake for 20 minutes. Remove weights and parchment; set aside.
Meanwhile, place stale bread in a food processor and pulse into crumbs. Measure out 4 ounces (2 1/2 cups) of the breadcrumbs and set aside. In a large bowl, whisk together golden syrup, egg, lemon zest, and 1 tablespoon of the lemon juice. Mix in breadcrumbs. Pour mixture into tart shell and spread evenly.
Place on a baking sheet and bake 40 to 50 minutes until the filling is set (a slight jiggle and some bubbling is OK.) Cool completely on a wire rack, then trim crust edge. Serve topped with whipped cream (if desired). Can be stored at room temperature for 2 days or in the fridge for 4 days.
Per serving: 519 calories, 14 grams fat (8 grams saturated fat), 51 milligrams cholesterol, 580 milligrams sodium, 93 grams total carbohydrates, 2 grams fiber, 8 grams protein.
