1. In bowl, whisk mustard while slowly pouring oil from tuna can until thick, glossy mayonnaise forms. (Tip: add as much oil as desired, reserving additional oil for another use).
2. Season mayonnaise with pepper and salt.
3. Add tuna into bowl and stir gently just to combine.
4. Distribute tuna mixture evenly onto sourdough slices.
5. Top with sun-dried tomatoes, onions, and dill sprigs.
6. Serve immediately.