1. Preheat the oven to 350 F and lightly grease a 9″ x 13″ baking dish (3 quarts). Bring a large pot of salted water to a boil. Add the noodles and cook until al dente, about 4 minutes, or as per package directions. Drain the noodles and then rinse with cool water. Set aside.
2. Melt the butter over medium-high heat in a large saucepan on the stove. Once melted, add the onions and sauté for 3-4 minutes, or until translucent. Add the garlic and sauté for another minute. Add the flour, paprika, ground mustard, salt and pepper. Stir for 2 minutes or until the mixture thickens. Slowly pour in the milk, whisking continuously, until thick and bubbly, about 3 minutes. Add in the sour cream, and stir to combine. Remove the pan from heat.  Add the peas, tuna, cheddar cheese and diced olives, if using. Stir to combine. Remove the pan from heat.
3. In a small bowl, combine together the Panko crumbs with the melted butter and stir to combine. Sprinkle extra cheddar cheese over the top of the casserole, followed by the Panko crumbs, covering the top completely. Bake the casserole in the oven, uncovered, until bubbly, about 25 minutes. During the last few minutes of baking, switch the oven to the broiling setting and broil for 2-3 minutes or until golden brown on top. Remove and serve immediately.

Leftover casserole can be stored in the fridge, in an airtight container, for 3-4 days. Whole, unbaked casseroles can be stored in the freezer for up 2 months. Bake from frozen for 45 minutes, covering the top with aluminum foil during the last half.