Preheat oven to 350 F. Arrange walnuts in a single layer on a sheet pan and toast until lightly browned, for about 5-7 minutes. Cool.

Spray a medium sized nonstick saucepan with canola cooking spray. Add onion and saut over medium heat until just softened. Add couscous and cook until lightly browned. Add 2 1/2 cups water; cover and reduce heat to simmer. Cook 8-10 minutes. Cool slightly. Add 1/2 cup of the walnuts, tuna, olives, capers, parsley, lemon zest, and lemon juice. Season to taste with salt and pepper.

Wash frisee, separate leaves and toss with vinaigrette. Divide evenly between 4 plates. Sprinkle with remaining 1/4 cup walnuts.

Stack slices of tomatoes, in alternating colors in the centers of the 4 plates, on top of frisee.

Fill four 8-ounce ramekins (or custard cups) with couscous mixture and pack tightly. Invert ramekin on top of tomato slices to unmold. Garnish with reserved walnut pieces and sprig of frisee. Serve immediately.