Split the bagels and spread each of the cut sides with butter and a little mustard. Separate the slices of pastrami, sandwich them between the bagels and put them on a sheet of double thickness kitchen paper on a microwave-proof plate – the paper stops the bread from becoming soggy underneath and sticking to the plate. Microwave on 750 watts for 1 minute.
While the bagels are warming, slice the dill pickles lengthways. Take the bagels from the microwave and lift off their tops. Pile in the lettuce and sliced pickles, squirt in some more mustard and enjoy straightaway while they’re still warm.