Bring a large pot of salted water to a boil. Cook the elbow macaroni until just shy of al dente, about 5 minutes.
In the meantime, heat the oil in a large skillet over medium-high heat, then add the celery, carrot, bell pepper and red onion. Season with a pinch of salt and saute until the veggies begin to get tender, about 5 minutes. Add the garlic and saute until fragrant, another 30 seconds.
Add the ground beef and brown it with the veggies, breaking it up with a wooden spoon. Season with the cumin, chili powder and a pinch of salt; toss the beef and veggies around in the skillet another minute or two. Add the diced tomatoes and stir into the mix. Transfer the mostly-cooked elbow macaroni to the skillet and stir to combine.
Transfer the chili mac to any casserole dish you want! Sprinkle with the mozzarella and bake, uncovered, until the cheese is browned and bubbly, 30 to 40 minutes.