Preheat your oven to 375°F (190°C). Grease a baking dish with cooking spray or butter and set it aside.
Cook the Kraft Macaroni and Cheese according to the package instructions, but reduce the cooking time by 2 minutes. We want the macaroni to be slightly undercooked as it will continue to cook in the oven.
In a skillet over medium heat, melt the butter. Add the diced onions and cook until they become translucent and slightly caramelized.
Add the frozen peas and cook for a few minutes until they are heated through. Remove from heat and set aside.
In a large mixing bowl, combine the cooked macaroni, the contents of the Kraft Cheese Sauce packet, sautéed onions and peas, drained tuna, and ½ cup of shredded cheddar cheese.
Stir everything together until well combined. Season with salt and pepper to taste.
Pour the mac and cheese mixture into the prepared baking dish, spreading it out evenly.
Sprinkle the remaining ½ cup of shredded cheddar cheese over the macaroni mixture.
In a small bowl, mix the breadcrumbs with a tablespoon of melted butter until well coated. Sprinkle the breadcrumb mixture evenly over the cheese.
Place the baking dish in the preheated oven and bake for approximately 20-25 minutes, or until the cheese is melted and bubbly, and the breadcrumbs are golden brown.
Remove the casserole from the oven and let it cool for a few minutes. Serve the Kraft Mac and Cheese Tuna Casserole hot, and garnish with fresh herbs if desired.