Preheat the oven to 225 degrees F. Fit a baking sheet with a wire rack.
Season the tenderloin with the granulated garlic and plenty of salt and pepper. Transfer to the wire-racked baking sheet and place on the middle rack in the oven. Roast low and slow until the internal temperature registers 120 degrees F, 2 to 2 1/2 hours.
Remove the tenderloin from the oven, cut off the butcher's twine and coat in the softened butter. Set the oven to broil. Broil each side until crusty and golden, about 30 seconds a side. Set aside to cool, reserving any juices for later.
Slice the tenderloin as thinly as possible. We are looking for a smooth bite down here, so thin slices are key. Pour the accumulated juices over the slices and hit with more salt and pepper.
Cut off the top crust of the boule about 1 inch down to create a lid. Hollow out the rest of the bread into a bread bowl. Smear the mustard over the bottom of the bread bowl. Next, spread with the Mushroom Duxelles. Then, add the sliced tenderloin; we are looking for a lot of height here, so ribbon those slices! Next comes a layer of baby spinach. Close with the top of the bread bowl and completely wrap in 3 layers of foil.
Place a heavy wooden cutting board on top and use books, weights, giant cans, etc. to press down on the sandwich evenly. We are looking for maximum compression here, so 50 pounds is the goal. Just make sure the weight is distributed evenly. Press for a minimum of 4 hours at room temperature.
Unwrap, slice into wedges and serve.
Add the mushrooms, garlic and shallot to a food processor and pulse a few times until coarsely chopped.
Heat the butter in a 12-inch skillet over medium-high heat until melted. Add the mushroom mixture and stir. Cook, stirring every couple of minutes, until the mushrooms begin to brown. Season with salt and pepper, deglaze with the sherry, then add the fresh thyme. Set aside to cool.