Whisk the mustard, lemon juice and a pinch each of salt and pepper together in a small bowl until combined. Toss the tomatoes, mushrooms, olives, capers, beans, tuna and celery together in a medium bowl just to combine and lightly break up the tuna. Add the dressing and toss to coat. Adjust the seasoning with additional salt and pepper.
Divide the greens among 4 plates and spoon the tuna mixture on top. Serve with warm pita bread and lemon wedges.
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