Place the button mushrooms in a ceramic bowl. Whisk the olive oil, lemon juice, Dijon mustard and salt and freshly ground black pepper in a bowl then pour over the mushrooms. Stir to coat. Cover and allow to marinate 20-30 minutes or until tender.
Add the semi-dried tomatoes, char-grilled eggplant, roasted capsicum and toasted pine nuts. Spoon into baby cos lettuce leaves, crumble over some feta and serve.