Lemon Myrtle is an Australian herb. It's quite difficult to get your hand on, but if you can it is fairly versatile.
Lemon Myrtle tends to come as a coarse, pale green powder. Its aroma and flavour is lemony and tangy like lemongrass and lemon verbena, but not especially strong. There is perhaps a hint of kaffir lime leaf and eucalyptus as well.
You can use lemon myrtle in marinades and rubs where you have some lemon juice, rubbing in about ½ tsp per 500g of chicken or meat. It is best chosen for grilling or barbecueing rather than for longer cooked stews, as the lemony notes can get evapourated, so leaving behind bitterness rather than citrus flavours. Similarly, add to your pancake batter for a light lemony taste that's great for breakfast.