Step 1Place butter, egg, water and dash salt in food processor work bowl fitted with metal blade. Pulse until butter is coarsely chopped. Spoon flour evenly over butter. Process just until dough is crumbly but not in ball (over-processing will toughen dough). Transfer dough to large plastic food bag. Work through bag to gather dough into ball, then flatten into disk. Place in another food bag, then refrigerate at least 2 hours or up to 2 days. (Dough may also be frozen up to 1 month; thaw slowly in refrigerator.)
Step 2Let chilled dough rest at room temperature 10 minutes before rolling. Roll dough on floured surface into circle about 1/8 inch thick. Transfer to 11-inch tart pan with removable bottom. Press into place without stretching dough. Trim pastry edge for 1-inch overhang. Reserve any pastry scraps for patching later. Fold overhang inside to make sides doubly thick. Press sides of dough firmly into place, pushing crust up 1/4 inch above edge of pan to allow for any shrinkage during baking. Cover loosely with plastic wrap and refrigerate until dough is firm, about 1 hour. Line shell with parchment paper or foil; fill with dried beans.
Step 3Set tart pan on baking sheet and bake on lower rack at 400 degrees until dough is starting to set, about 12 minutes. Carefully lift out paper and beans; continue to bake until lightly browned, about 12 minutes more. Patch any cracks with thin patches of reserved dough; it is not necessary to bake patches. Crust can be filled while still warm from oven, then baked as directed.
Step 4Makes 1 (11-inch) tart crust.
Step 1Cook bacon in 10-inch skillet over medium-high heat, stirring often, until brown but not crisp, about 6 minutes. Set aside on paper towels to drain. Discard grease; do not wipe out pan.
Step 2Melt butter in same pan over medium heat. Add leeks. Cook until softened but not browned, stirring often, about 8 to 10 minutes. Set aside.
Step 3Put eggs, cream, milk, nutmeg, salt and pepper to taste in bowl. Whisk until well mixed. Stir in bacon and leeks. Taste and adjust seasoning. Set tart pan on baking sheet and ladle bacon mixture into warm crust; fill to within 1/8 inch from top (you may not use all of filling). Dot top with diced tomato.
Step 4Bake at 300 degrees in lower area of oven until set and browned and puffy, about 50 to 60 minutes. For more browning, place under broiler 2 minutes. Let rest on rack at least 15 minutes before serving. To serve, remove tart from pan. Slide to serving platter. Cut into wedges. Serve warm or at room temperature (it can be held at room temperature a few hours.)