1.Preheat oven to 220°C/200°C fan-forced.
2.Quarter capsicums; discard seeds and membranes. Roast, skin-side up, until skin blisters and blackens. Cover capsicum pieces with plastic or paper for 5 minutes; peel away skin.
3.Place eggplant and mushrooms on oiled oven tray. Roast about 15 minutes or until tender; cool.
4.Cut shallow lid from top of loaf; remove soft bread inside, leaving 2cm-thick shell.
5.Spread half the combined mustard and mayonnaise inside bread shell and lid. Place beef inside bread shell; top with mushroom then capsicum and eggplant, pressing layers down firmly. Spread with remaining mayonnaise mixture; top with lettuce. Replace lid; press down firmly.
6.Wrap loaf tightly with kitchen string and plastic wrap; refrigerate 2 hours or until required.