Preheat the oven to 350 degrees F.
Lightly grease a 9x13 baking dish and set it aside.
Add water to a medium pot and season it with salt. Bring the water to a boil. Stir in the pasta and boil for about 8 minutes, until just tender.
Transfer it to a colander, drain well and return to the pot.
In a large skillet over medium-high heat, add some olive oil, once hot add the diced onion, mushrooms, salt, and pepper. Stir and cook until the onions are tender. Add minced garlic and cook for 30 seconds. Set aside.
Next, add the beef to the skillet, and cook until brown in color. Season with salt, pepper, garlic powder, onion powder, and dried parsley. Break it with a wooden spoon while cooking.
Add Worcestershire sauce, tomato sauce, diced tomatoes, and Italian seasoning. Stir to combine.
Transfer the beef mixture to the prepared casserole. Sprinkle one cup of cubed cheddar cheese on top.
Add the sauteed mushrooms, on top. Then, toss in the pasta shells, and another 1 cup of shredded cheese. Toss to combine, and top with remaining 1 cup of shredded cheese.
Bake for 30 minutes, or until the cheesy is bubbly.
Serve warm topped with chopped pickles, iceberg lettuce, tomatoes, and sesame seeds.