The filling only gets better as it sits: keep leftovers in an airtight container in the fridge for up to 4 days, or freeze for up to 3 months and reheat gently with a splash of water. Brown or green lentils hold their shape best; red lentils cook down into a softer, more spreadable filling. For a gluten-free version, swap the soy sauce for tamari and use gluten-free buns.