For the vinaigrette, combine lemon juice, olive oil, mustard, dill, salt, and pepper in a mason jar. Seal jar and shake vigorously until emulsified.
Drain tuna and divide evenly. Place on romaine lettuce leaves and drizzle with vinaigrette. Use optional toppings.
To prepare ahead of time for weekly meal planning, mix tuna with dressing and toppings and combine with romaine lettuce leaves right before serving/eating.