In a bowl, combine the flour and salt. Add the butter and rub between your fingertips, incorporating the mixture as you work, until the butter forms flaky pieces. Gradually add the water and combine until just moistened. Pieces of butter will remain and the dough will be soft and sticky.
On a lightly floured surface, roll out the dough into an 8 x 13-inch (20 x 33 cm) rectangle. Brush off any excess flour. Fold the dough into thirds lengthwise, then fold it onto itself to create a square. Cover in plastic wrap and refrigerate for 1 hour or until firm.
In a pot of salted boiling water, cook the carrots until tender. Drain and let cool.
In a skillet over medium heat, soften the onions in the oil. Let cool.
In a large bowl, combine the crackers, eggs and milk. Add the onions and the remaining ingredients except the carrots. Mix thoroughly using your hands. Season with pepper and set aside.
With the rack in the middle position, preheat the oven to 400°F (200°C). Butter a 6-cup (1.5 litre), 4 x 10-inch (10 x 25 cm) loaf pan. Line the pan with parchment paper, letting the paper hang over two sides.
In a bowl, whisk together the egg and milk. Set the egg wash aside.
On a lightly floured surface, roll out three-quarters of the dough into a 12 x 17-inch (30 x 43 cm) rectangle, 1/8 inch (3 mm) thick. Line the loaf pan with the dough and trim the excess down to 1 inch (2.5 cm), letting it drape over the edges. Roll out the remaining dough into a 5 x 11-inch (13 x 28 cm) rectangle.
Spoon a quarter of the meat mixture into the loaf pan and cover with 2 carrots. Continue filling the pan with the meat and carrots, ending with the meat. Brush the pastry edges with the egg wash and cover with the second piece of dough. Cut several vents in the dough. Press the edges together to seal and brush with the egg wash. The dish can be frozen at this stage and baked later, if desired.
Place the meatloaf en croûte on a baking sheet. Bake for 1 hour and 30 minutes or until a meat thermometer inserted into the centre of the loaf reads 180°F (82°C). Let rest 10 minutes before unmoulding. Slice and serve with bacon and red wine sauce (see recipe).