Cook elbow pasta in a pot of salted, boiling water, with a teaspoon of oil, for 8 to 12 minutes until al dente(tender). Drain and rinse under cold water.
Place the bell pepper, red onion, lemon juice, and salt in a large bowl, stir to combine, and set aside for 15 minutes to marinate.
Transfer the drained pasta to the bowl of marinated vegetables.
Place the Mayonnaise, Plain yoghurt, mustard, and black pepper in a medium bowl and whisk to combine. Add drained Tuna to mixture.
Transfer Tuna mixture to bowl with Pasta and mix well. Season with salt and black pepper to taste. Serve Cold.