How to Make It
Heat a grill pan. Rub the steak all over with 2 tablespoons of the oil and season with salt and pepper. Grill over moderately high heat, turning once, until medium-rare, about 3 minutes per side. Transfer to a cutting board and let rest for 10 minutes.
In a bowl, toss the spinach with the lemon juice and the remaining 1 tablespoon of oil. Season with salt.
Spread a heaping tablespoon of the shrimp-chile jam on the bottom half of each roll and spread 1/2 tablespoon of the butter on each of the top halves. Spread with mustard, if using. Mound the spinach mixture on the rolls, top with the steak and serve.
Pleasantly sweet Portuguese rolls are available at specialty food stores. Use this recipe for Dried Shrimp-Chile Jam.