Pound the flatiron steak with a smooth meat mallet or heavy rolling pin to tenderize and even its thickness.
Pour the pickle juice into a shallow container, then add the thinly sliced red onion and a little black pepper.
Add the steak to the pickle-onion mixture, seal the container, and marinate in the refrigerator for 3–5 hours.
Heat a heavy-based frying pan over medium-high until hot.
Remove the steak from the marinade and pat it dry with paper towels.
Rub the steak with the tablespoon of oil and season lightly with salt and additional black pepper if desired.
Flash-fry the steak in the hot pan for about 2 minutes per side for a medium-rare to medium finish, then transfer to a plate and tent with foil to rest.
Cut each mini dill pickle in half. Thinly slice the rested steak across the grain, then cut each slice in half into small pieces suitable for skewering.
Thread a small piece of beef and half a dill pickle onto a cocktail stick or small skewer to make each skewer.
Refrigerate the assembled skewers until needed, then serve with sweet mustard and a garnish of fresh dill.