Step 1 Put the mustard greens in a pot over high heat with the water that clings to the leaves plus one-half cup. Sprinkle with one-half teaspoon salt, pepper to taste, garlic and the pepper flakes and cover. After the leaves have collapsed, reduce the heat to medium and cook until they're tender when you taste one, about 7 minutes. Drain, then squeeze the excess water out of the greens. Put them in a bowl and season with additional salt, if needed, and pepper sauce or vinegar to taste.
Step 2 Slather the outside of the bread with olive oil. Cover one slice of the bread (the dry side) with cheese, pile on a half or a third of the greens, and add the pepper strips. Spread the top slice with Dijon mustard, then cover.
Step 3Cook in your panini maker or in a skillet until the bread is crispy and the cheese melts. When a wave of melted cheese hits the hot surface, there's a bonus tang, but don't let it burn. Slice it diagonally; it's easier to eat that way and it looks jaunty too.