For the mousse, mix together the ricotta, egg yolk, and sugar, beating well. Add the lemon juice and zest and then the finely chopped watercress. Stir in the gelatin and season with salt and pepper. Whisk the egg whites until they are stiff, and fold in.
Line small ramekins with plastic wrap and fill with mousse bringing the wrap over the top of the mousse. Chill until the mousse is just set. Unmold and serve.