Preheat your oven to 375°F (190°C). Grease a 9x13-inch baking dish.

Fill a large pot with salted water, bring to a boil, and cook the pasta al dente for 8-10 minutes.

Heat olive oil in a skillet, sauté chopped onion for about 4 minutes, then add minced garlic and cook for an additional minute.

In a mixing bowl, combine sautéed onions and garlic with Greek yogurt and mustard, stirring until smooth. Adjust thickness with reserved pasta water if needed.

Fold in drained tuna, cooked pasta, and frozen peas into the Greek yogurt mixture until well-coated.

Transfer the mixture to the greased baking dish, top with remaining cheeses and optionally breadcrumbs.

Bake uncovered for 20-25 minutes until cheese is bubbly and golden brown.

Let the casserole rest for about 5 minutes before serving.