Form a square of sausage stuffing. Place 1 tbsp butter and 1 tsp olive oil in Small Fry (or another skillet) on medium heat until butter begins to sizzle.
Add the slab of stuffing and brown on both sides, making sure to cook slowly enough so the center warms through.
Cut off the furry bottom and toughest parts of the top of the leek. Slice in half lengthwise and then crosswise into 2-inch segments. Rinse the leeks of any grit and dry them well — you can use a salad spinner.
Heat the remaining butter and oil in Deep Cut (or another sauté pan), and cook leeks on medium-low until they melt, about 20–25 minutes. Remove pan from heat, and stir in Dijon mustard.
Toast bread of your choice and butter both sides. Spread leek mixture on bottom slice, then add the slab of sausage. You could also add a piece of crisp Bibb lettuce here.