Make the dressing: Whisk the mustard, lemon zest and lemon juice in a small bowl. Slowly whisk in the olive oil until well blended. Whisk in the dill, 1/2 teaspoon salt, and pepper to taste.
Toss the tuna, chickpeas, cucumber, scallions and tomatoes with 1/4 cup of the dressing in a medium bowl.
Heat a grill pan over medium-high heat. Lightly brush both sides of the pita with olive oil and season with salt and pepper. Cook until golden, about 1 minute per side. Cool slightly and cut into wedges.
Toss the romaine with the remaining dressing and divide among bowls. Top with the tuna salad and serve with the grilled pita.