How to Make It
In a chilled bowl, whisk the 3 cups of flour with the salt. Scatter the butter over the flour. Add the lemon juice and 1 cup of cold water and stir with a wooden spoon just until the flour is moistened; add 1 to 2 tablespoons more water if needed. The butter will still be very visible.
Gather up the dough and turn it out onto a floured surface. Spread it with the heel of your hand into a rough 8-by-12-inch rectangle. Brush any flour off the surface of the dough. Using a pastry scraper, fold the dough in thirds (as you would fold a letter) to make a rough 8-by-4-inch rectangle.
Lightly flour the dough and pound it with a rolling pin to flatten it slightly. Roll out the dough to an 8-by-20-inch rectangle. Brush off any excess flour. Using the pastry scraper, fold in the bottom end and the top end so they meet in the center without overlapping, then fold the rectangle in half where they meet; this is called a "double fold." Roll out the dough to an 8-by-20-inch rectangle 2 more times, flouring as necessary and doing a double fold each time. Wrap the dough in plastic and refrigerate for 1 hour, or until well chilled.
Cut the dough into 3 equal pieces. Wrap 2 pieces in plastic and refrigerate or freeze for later use. On a lightly floured surface, roll out the dough to an 8-by-18-inch rectangle a scant 1/8 inch thick. Transfer to a parchment-lined baking sheet and freeze for 5 minutes, or until firm. Use a ruler and a very sharp knife or pastry wheel to neatly trim the edges of the dough, forming it into a neat 7 1/2-by-17-inch rectangle.
Brush 2 inches all around the edge of the rectangle with water. Fold in 1 1/4 inches of the edge all around the tart. Press lightly to seal. Using the ruler and knife, trim the edges all around, so the 2 layers of dough can be seen on all 4 sides. Using a fork, prick the bottom of the dough all over to prevent it from rising. Brush the rim lightly with the egg wash and, using a paring knife, score it with shallow crosshatched lines for decoration. Freeze the tart shell for 1 hour.
Preheat the oven to 400°. Position a rack in the lower third of the oven. Bake the tart shell until golden and the rim is well risen, about 35 minutes; if the bottom puffs during baking, press it down with the back of a fork.
While the shell is still warm, spread 1/4 cup of the raspberry jam over the bottom in an even layer. Set aside 3/4 pint of the best-looking berries. Spread the remaining raspberries in the tart shell in an even layer and lightly crush them with a fork. Transfer the tart on the parchment paper to a rack to cool.
In a small saucepan, melt the remaining 3 tablespoons of raspberry jam. Spoon half of it over the crushed berries. Arrange the remaining whole raspberries in neat rows on top and carefully brush with the remaining melted jam. Let the tart cool, then cut into strips and serve with vanilla ice cream, whipped cream or crème fraîche.