The combination of egg mayonnaise and crunchy, peppery cress is hard to beat, but I find standard sprouting cress a nuisance; bite into it at the wrong angle and it comes out of your sandwich in a soggy tangle and drips everywhere. So here I've substituted a packeted watercress salad mix, chopped. The same peppery taste but with more volume and crunch, and a bit more manageable.
As watercress and any green leaves will go soggy, these sandwiches need to be eaten within hours but you can prepare the filling beforehand and put them together at the last minute.
Gardeners willl be fortunate enough to be more creative regarding cress and cressy flavours, such as land cresses or nasturtium leaves which give a salad or sandwich a wonderful 'cressy' peppery flavour. In fact if you're serving these up at your garden party you could cut them up into dainty finger sandwiches as suggested above, and even garnish with a green sprig and a nasturtium flower.