First things first, bring a large pot of salted water to boil. Add your whole wheat pasta to the pot and cook according to the package directions until al dente. Once done, drain and set aside. This will not only help the pasta maintain its firmness but also absorb the casserole's flavors.

In a large skillet, heat a splash of olive oil over medium heat. Add the chopped onion and sauté until it becomes translucent. This should take around 5 minutes. Next, toss in the diced bell peppers and cook for an additional 3-4 minutes until they soften. Then, add the frozen peas and spinach, stirring until the greens wilt and everything is well combined.

In a separate mixing bowl, whisk together the Greek yogurt, chicken stock, whole milk, Dijon mustard, Italian seasoning, crushed red pepper flakes, and black pepper. This creamy mixture is what binds everything together in the casserole.

In a large mixing bowl, combine the cooked pasta, sautéed vegetables, and drained tuna. Pour the yogurt sauce over the top and mix gently until every ingredient is evenly coated. Collective harmony is what we aim for here!

Preheat your oven to 350°F (175°C). Transfer the mixture into a greased baking dish, spreading it out evenly. To top it off, sprinkle shredded cheddar cheese over the casserole. Now it’s ready for baking. Cover with foil and bake for about 20 minutes. Remove the foil and let it bake for an additional 10 minutes until the cheese is bubbly and golden brown.

After baking, let the casserole cool for a few minutes before serving. This waiting period allows the flavors to settle, making each forkful even tastier. Serve it warm and enjoy!