Preheat the oven to 350 degrees. Spray a 9 x 13” inch casserole dish with cooking spray. Bring a large pot of salted water to a boil. Cook pasta according to package directions for al dente, drain.
While the pasta is cooking, heat oil in a large skillet over medium-low heat. Add onions and cook until soft (about 5 minutes). Stir in the garlic and cook for 30 seconds. Add the ground beef and cook while breaking up into chunks with a spoon or spatula until browned. Season with salt and pepper. Stir in tomato paste, and then add the tomatoes and mustard. Let the mixture bubble gently until it is slightly thickened (about 2 minutes).
Toss the meat mixture with the cooked pasta and spread it into the prepared baking dish. Top with cheddar cheese. Place the baking dish into the oven and bake until the cheese has melted (about 15 minutes). Remove from the oven, sprinkle the chopped dill pickles over the top and serve.