Finely chop the Brussels sprouts and water chestnuts with a knife (or in a food processor) and into small pieces.
Place in a bowl with lemon juice and toss to coat (this should prevent the leaves from losing their bright color). At this point you can cover the brussel sprouts and refrigerate for several hours until you're ready to cook them.
Heat butter and oil in a saute and pan over medium heat. Saute the brussel sprouts for 2 minutes. Add the salt, garlic, zest and poppy seeds and continue to cook an additional 2 minutes.