Makes 2

Of all the high-street sandwiches, crayfish and rocket, or watercress, is about my favourite. It's an ingenious freshwater version of the prawn cocktail, especially when watercress rather than rocket is used, calling to mind clean running water, a habitat for both the crayfish and the watercress.

Sadly, though, many modern watercress farms are doing more harm to our waters than good. And so are the rapidly breeding American signal crayfish, which are taking over our waterways and munching anything that gets in their way, including fish eggs. It makes sense to eat these pests rather than imported crayfish (and it's illegal to catch and eat the threatened native crayfish) as it helps keep down the numbers of these predatory creatures that are driving away our native river dwellers. Unfortunately most of the crayfish consumed in this country is imported in brine and does nothing to help control the onward march of this freshwater menace. Our restaurant, J Sheekey, uses a man who fishes for crayfish on the upper Thames north of Oxford, and supplies a handful of other leading restaurants who cook them with a clear conscience, knowing they are helping to keep rivers pest free. Anyone can buy them for £19.50 a kilo, from ClubChefDirect (01275 475252/, a gourmet delivery service endorsed by chefs.

Cook live freshwater crayfish in salted water with lots of aromatic flavourings such as dill, fennel seeds, peppercorns, or as we do, in beer and wild fennel. There isn't much meat on them, but the shells make an excellent bisque.

4 slices of hand-cut bloomer-style bread
Softened butter
About 80g crayfish meat
About 20-30g watercress, washed, dried and roughly chopped
2tbsp good quality mayonnaise
1/2tbsp chopped dill or fennel
Salt and freshly ground black pepper

Mix the mayonnaise with the chopped dill and crayfish and season. Butter the slices of bread and spread the crayfish mixture on two of the slices. Scatter the watercress on the crayfish mixture and sandwich together with the other slice of bread. Serve cut in half as it may get messy into four.