Mix the beef mince and ‘nduja in a bowl and season. Form the mix into a patty about the same size as your bread slices and pop onto a plate and chill whilst getting everything else together.
Put the sliced shallot into a cold pan with half a tablespoon of oil and pop into Roccbox. Turn the flame down to is lowest setting and cook for around 10 minutes, shaking and stirring occasionally. Season the onions and set to one side.
Turn the flame back up to full and give the pan a quick wipe out before allowing to heat up in Roccbox for 30 seconds. Rub the patty with a little oil and add to the hot pan. Cook for 4 minutes in total, flipping and rotating as required.
Split the dijon between the 2 slices of bread and top each slice with the onions. Add 2 slices of Emmental and half the blue cheese to one slice of bread and sit the patty on top – pressing down as your do so to fill the gaps (the patty should be quite malleable). Add the remaining cheese on top of the patty and top with the second slice of bread. Brush one side of the sandwich with melted butter.
Your pan should still be hot enough to add the sandwich buttered side down to the pan. Spread the other side of the sandwich with butter, turn Roccbox down to its lowest setting and return the pan to Roccbox.
Cook for 2-3 minutes rotating and turning as required or until the cheese is gooey and the bread is golden brown and crispy.