Whisk mayo, miso, and lemon juice in a medium bowl, breaking up the miso with the whisk. Add gravy and whisk until almost completely smooth. Stir in chives and thyme, and season to taste with salt and pepper. Can be made one day in advance.
Swipe a generous amount of butter on each side of the bread slices. Toast the bread in Small Fry (or another skillet) on medium-high heat until both sides are golden brown, 1 to 2 minutes per side. Let the bread cool slightly before building the sandwich.
Slather both slices of bread with a layer of gravy mayo. Pile on the rest of the ingredients in whatever manner you see fit. Cut sandwich in half, if you’d like.