Yield:  About 4 servings

For Pork:
1 pork tenderloin (about 1 pound)
1/2 cup kosher salt, plus additional to season
1/2 cup sugar
2 bacon slices, cut crosswise into 1/4-inch strips
5 ounces baby spinach, chopped
2 garlic cloves, minced or pressed
1 tablespoon coarse-grain mustard
Freshly-ground black pepper
1 hard-boiled egg, quartered lengthwise
1-2 tablespoons high-heat vegetable oil

For Sauce:
2 hard-boiled eggs, diced fine
2 tablespoons olive oil
2 tablespoons mayonnaise
1 tablespoon dijon mustard
1 tablespoon fresh lemon juice
1 tablespoon drained capers
1 1/2 tablespoons finely sliced chives
1/4 cup finely diced cornichons or dill pickles
1 teaspoon black pepper
Pinch of sugar

To Serve:
1 large baguette, cut into 4 pieces and sliced open
8-12 large leaves butter lettuce, rinsed and dried
    

Fill a large pot or bowl with the 6 cups of cold water.  Add the kosher salt and sugar.   Stir until the salt and sugar have dissolved.  Add the pork tenderloin, making sure it is fully immersed in the brine--you may need to add up to 2 cups more cold water or weigh it down with a plate depending on the size of your brining pot.  Set aside to brine for 45 minutes.

Remove the pork from the brine.  Pat it dry and set it aside while you prepare the stuffing.

Cook the bacon in a large, heavy ovenproof skillet over medium heat, stirring occasionally, until the fat is rendered and the bacon is golden and only slightly crisp.

Add the spinach and garlic and cook, stirring often, until the spinach is wilted and tender, about 3 minutes. Stir in the grainy mustard and season to taste with salt and pepper.  Set aside.

Preheat the oven to 425°F with a rack in the middle.

Place the tenderloin on a work surface with the thinner, pointed end nearest you. Starting at the top (thicker part), make a lengthwise incision down the center of tenderloin, cutting two thirds of the way through the thickness of the meat.  You will need to adjust the depth of the incision as the tenderloin tapers (be careful not to cut all the way through).  Open the tenderloin and, turning your knife horizontally, cut the tenderloin open on either side like the flaps of a book jacket, being careful not to cut all the way through.  Place a large sheet of plastic wrap over the butterflied pork and pound the pork to about  1/3-inch thickness.  Remove the plastic wrap and season the inside of the tenderloin generously with pepper (do not salt the pork as it will already be salty from the brine).

Turn the tenderloin so that a long side is nearest you, then spread the spinach filling evenly over the opened tenderloin, leaving a 1-inch border along the edge farthest from you.  Arrange the egg quarters, end to end, along the edge nearest you.

Roll up the tenderloin tightly, then tie with kitchen twine at 1 1/2-inch intervals.  Pat the tenderloin dry, removing any spinach filling that has escaped.

Heat 1 tablespoon of the vegetable oil in the cleaned skillet over high heat until it shimmers. Sear the tenderloin until it is golden brown on all sides, 8 to 10 minutes.  Transfer the skillet to the oven and roast until an instant-read thermometer inserted diagonally 2 inches into the thickest part of the roulade registers 135°F, 10-15 minutes (the pork will continue to rise in temperature out of the oven).

Transfer the tenderloin to a platter to rest, uncovered, for at least 30 minutes.

While the pork is resting make the sauce: Place the diced  hard-boiled egg in a small bowl.  Stir in the remaining sauce ingredients.  Taste and adjust seasoning.

When you are ready to assemble the sandwiches, cut the string from the tenderloin, then slice it thinly.  Place some of the butter lettuce on the bottom half of each section of baguette.  Spoon a generous amount of sauce over the lettuce.  Arrange the sliced roulade over the sauce and place the baguette tops over the pork.

*Note: The pork roulade can be served cold or at room temperature.  If you do not wish to eat it after it has rested for 30 minutes, store the unsliced roulade in the fridge, tightly wrapped.  Store the sauce, covered, in the fridge.  The sauce and roulade will keep for up to 2 days.  Slice the roulade just before serving to keep it from drying out.