Bring a large pot of salted water to a boil and cook pasta until al dente, about 8 to 10 minutes. Drain, reserving ½ cup pasta water.

In a skillet over medium heat, warm 1 tablespoon of olive oil. Add chopped onion and sauté for about 4 minutes until soft. Stir in minced garlic and cook for an additional minute until aromatic.

In a mixing bowl, combine the sautéed onion and garlic with Greek yogurt, mustard, and half of the shredded cheese. Stir until smooth.

Gently fold in drained tuna, cooked pasta, and frozen peas, ensuring everything is well coated with the sauce.

Preheat oven to 375°F (190°C). Transfer mixture to a greased baking dish, top with remaining cheese and breadcrumbs. Bake for 20 to 25 minutes until golden brown.

Remove from oven and let it rest for 5 minutes before serving. Add a squeeze of fresh lemon juice if desired.