Prep: Preheat the oven to 375°F. Coat one 9x13 inch casserole dish with cooking spray (or two 8x8 dishes and freeze one for later).
Boil Pasta (& Reserve Water): Bring a large pot of water to a boil and salt the water with 2 teaspoons salt. Add the macaroni and cook until tender but still a bit firm. Important: Before draining, remove 3/4 cup of pasta water and set aside. Drain pasta in strainer/colander and set aside.
Blend Sauce: While the pasta is boiling, place the sweet potato flesh (orange part), milk, pasta water, yogurt, salt and pepper (and optional mustard) into a blender. Blend until smooth (this hides the veggies from keen eyes and helps thicken the sauce).
Finish Sauce: Then pour the milk mixture into the same pot you cooked the pasta in. Stir frequently over medium-high heat and cook until the mixture is hot and nearly simmering. Remove from the heat and stir in the cheese a little bit at a time (use a figure 8 stir pattern to prevent clumping). Stir until melted and smooth.
Add Pasta: Add the cooked macaroni to the cheese sauce and stir to combine. You can totally eat and enjoy at this point if you want OR transfer the macaroni and cheese to the baking dish(es).
Make Topping: Combine bread crumbs, oil, and optional parsley in a small bowl. Sprinkle evenly over the top of the macaroni and cheese.
Bake: Bake for about 30-35 minutes, until the top is lightly browned, bubbly on edges, and hot in the middle. (This may take more like 20-25 minutes, if baking a half portion.) If you want it to brown up a little more on top, turn on the broiler for 1-2 minutes (watch closely!).
Freeze For Later: Follow Steps 1-6 (no need to preheat the oven). Do not bake the casserole. Tightly wrap the dish(es) in a few layers of plastic wrap or foil and press out any excess air. Label and freeze.
Prepare From Frozen: When ready to eat, thaw in the fridge overnight or use the defrost setting on the microwave. Bake as directed.