Heat the oven to 375° F. Coat a 9x13-inch casserole dish with nonstick cooking spray.
Bring a large pot of lightly salted water to boiling. Add pasta and cook 10 minutes, according to package directions. Drain.
While pasta cooks, heat a large nonstick skillet over medium-high heat. Add beef, breaking apart with a wooden spoon. Cook 5 minutes, then add onion, salt, and pepper. Continue cooking until no longer pink.
Transfer meat to a large bowl and mix in ketchup, mustard, and pickle relish. Add the drained pasta and stir.
Wipe out skillet and return to medium heat. Add butter and once melted, whisk in flour. Cook 2 minutes, then whisk in milk in a slow and steady stream. Cook 2-3 minutes, until slightly thickened. Remove from heat and add cheese and pumpkin, whisking to melt completely. Add salt and pepper to taste.
Pour sauce over meat and pasta and stir well to coat. Pour into prepared baking dish, making sure the meat is evenly distributed. Add sliced or diced tomatoes.
Melt remaining butter in a small microwaveable bowl, then whisk in panko. Sprinkle over the pasta.
Bake for 20 minutes, until golden. If desired, broil for 2 minutes to toast the panko.