Early in the mission when you very first encounter Brutes, go up the stairs to your left. Look at the holographic menus on the kiosk here and zoom in on the ostrich-like Moa head and cheeseburger symbol sign. There are two kiosks with these signs (one big and one small sign)along this walkway. They have a 7.77cr price for the burger if you are confused about which sign. Look at one or the other and it will pop Wiljax Brantly eventually. Wiljax Brantley owned a moa restaurant in Halo lore. This isn't the only such kiosk in the mission so you can unlock this at any of them presumably.