For the rice: bring to boil, reduce to a simmer and cook for 15 minutes. For this casserole I'll knock off 2-3 minutes off the cooking time so that the rice can finish cooking in the oven.

Preheat the oven to 375°. Heat up a large oven safe skillet over medium heat. Add the mirepoix and enough broth so that it doesn't stick. Add all of the seasonings. Stir until the vegetables are wilted and fragrant. This should take 10-15 minutes. If it takes longer, cover the skillet with a lid and let it cook uninterrupted for 5-7 minutes, that'll help the vegetables cook faster.

Add the butter and scatter the flour over the contents of the skillet. Use a sturdy utensil to mix the butter and flour around the veggies until it creates a light paste. Slowly add the whole milk while you stir. A thick and creamy sauce will form once you've added all the milk. Take a taste and see if any more seasonings are needed!

Add the frozen vegetables and the drained tuna into the creamy sauce. Use a wooden spoon to break it up so there aren't any huge tuna chunks. If the sauce has gone a bit thick and stodgy then pour some broth in to loosen it up a bit.

Add the cooked rice into the skillet. Stir to combine in thoroughly. Add the mustard and hot sauce then take another taste test and see if it needs any more salt, pepper, herbs, or acid (mustard/hot sauce). Smooth out the top and then bake it in the oven for 15 minutes. It is done when the top forms some nice crusty bits.

Let the casserole rest for a bit and then serve. Any leftovers can be kept in the fridge and make for great lunches! Enjoy!