Stir the fresh breadcrumbs with the whole milk until fully absorbed, 10 minutes, then mash to a smooth paste and set aside.
Remove and discard the bay leaves. Grind the marinated meat mixture once through a 1/4-inch (6 mm) plate into the chilled bowl.
Add the egg and the breadcrumb–milk paste to the ground mixture and mix vigorously with a wooden spoon or gloved hand until sticky and uniform, 1–2 minutes.
Preheat the oven to 325°F (165°C). Rinse the caul fat briefly under cool water, squeeze dry, and use it to line a 1-quart terrine or 9×5-inch loaf pan, letting a little overhang.
Pack the forcemeat into the lined pan, pressing to eliminate air pockets and smoothing the top; fold the overhang to cover and seal. Cover with the terrine lid or tightly with foil.
Set the pan in a deep roasting pan and pour in hot water to reach halfway up the sides. Bake until the center reaches 150°F/65°C and the juices run mostly clear, 75–90 minutes.
Remove the terrine from the water bath, uncover, and carefully pour off any surface fat. Lay a piece of parchment on the surface and press with a flat board or foil-wrapped cardboard weighted to about 2 pounds. Cool to room temperature, then refrigerate at least 12 hours, preferably 24.
Unmold, slice 1/2 inch thick with a sharp knife, and serve chilled or at cool room temperature with cornichons, Dijon mustard, and crusty bread.