Remove excess fat and tendons from the flank steak and divide into 2 equal pieces. Finely grind the ingredients for the espresso rub in a spice grinder. If you don't get roasted cumin or coffee beans, you can roast them yourself in the pan. Massage the rub nicely into the first piece of flank steak and leave to stand in the refrigerator for a few hours in a freezer bag. Mix the ingredients for the marinade well and let it steep in the fridge with the second piece of meat in a freezer bag.
For the potato salad, peel the potatoes or clean well if you prefer it with the skin on. Boil the potatoes, let them cool and cut into large pieces. Chop the bell pepper, parsley and onions and set aside. If you don't get pickled red onions, you can easily make them yourself. Roughly chop the onions and pour a boiling stock of red wine vinegar, salt, sugar and mustard seeds over them.
Mix the mayonnaise with the two types of mustard and a little stock of the onions to make a dressing. Now carefully mix in the potatoes and other ingredients. If the potatoes are still warm, the dressing will soak in better.
Pre-cook fresh corn on the cob. Mix all the ingredients for the marinade and let the corn on the cob steep for 1-2 hours. Brush with the marinade again and again when grilling.
Take the flank steaks out of the refrigerator 1-2 hours beforehand and bring them to room temperature. Bring the grill to a good heat and fry until crispy for about 5 minutes on each side, depending on the thickness. Then pull the steaks into the indirect heat and let them cook. Always turn it over so that the meat juice is evenly distributed. Then let it rest for 10 minutes under aluminum foil. Cut into thin slices across the grain and serve with the corn on the cob and potato salad.