Prepare couscous according to package directions, except omit olive oil, and add frozen vegetables to water before bringing to a boil. Once prepared, fluff couscous with a fork. Cool, uncovered, 10 minutes.

Meanwhile, in large bowl, whisk together olive oil, lemon juice, oregano and red pepper flakes. Add cooled couscous, tossing to coat evenly.

Gently stir in tomatoes and olives. Gently stir in cheese. Chill at least 2 hours before serving on lettuce leaves.