Place the green tea and sugar lumps in a teapot. Pour over a little boiling water and leave to steep for 5 minutes.
Stuff the mint and lemon verbena leaves into the pot, packing them in as tightly as you can. Add more sugar lumps to taste – the sugar enhances the flavour of the mint – and top up the pot with boiling water.
Place the teapot over a pan of boiling water or over a low heat on the hob, or simply cover with a tea cosy. Leave the tea to brew for 10 minutes.
Place 4 tea glasses on a tray. Pour some of the tea into a glass, then tip it back into the pot. Hold the pot high above the glasses and pour slowly so that bubbles form on top of the tea. Serve immediately.