Preheat the griddle or a large skillet. Heat the griddle or a large skillet to a medium heat. *The temperature setting will vary based on what griddle you are using, but for my 4 burner Blackstone, I started with the 2 outer burners on medium-high and the 2 inner burners on low.
Cook the bacon. Lay the bacon on the griddle first - it takes the longest to render. Cook until crisp, then remove and set aside, leaving the rendered fat on the griddle. If needed, you can move the bacon to the side of the griddle to continue to cook while you cook the other meats.
Boil the pasta. While the bacon cooks, bring a pot of salted water to a boil. Cook the spaghetti to al dente, then drain and set aside. Reserve about ½ cup of the pasta water before draining.
Sear the ground beef. On a hot section of the griddle, cook the ground beef, letting it develop a good sear before breaking it apart. Once cooked through, season with 1 teaspoon Cajun seasoning, then move to the side of the griddle (or remove from the skillet).
Cook the andouille sausage. In a separate section of the griddle, cook the sliced andouille until browned and slightly crisped. Move to the side of the griddle. (Or remove from the skillet and set aside.)
Cook the vegetables. Add the chopped onion, bell pepper, celery, jalapeño (if using), and mushrooms (if using) to the griddle in the residual fat left behind from the bacon and sausage. Cook until softened and lightly browned.
Combine the meats and vegetables. Add the cooked ground beef and andouille sausage into the vegetable mixture and stir to combine. Set this mixture aside on a cooler section of the griddle, or off to one side.
Cook the garlic. Clear a space in the center of the griddle and add 1 tablespoon of the reserved bacon/sausage fat, forming a small pool. Add the minced garlic to the fat and cook for about 1-2 minutes, until fragrant.
Toss the pasta with the garlic. Add the drained spaghetti directly to the garlic and toss together on the griddle to coat.
Add the meat and vegetable mixture. Add the cooked meat and vegetable mixture to the spaghetti and sauté everything together for about 2 minutes.
Make a well and build the sauce base. Make a "well" in the middle of the pasta mixture so you can see bare griddle in the center. Add the tomato paste to the well and cook for about 1 minute to knock off the raw flavor. Add the French onion soup to the well with the tomato paste and cook for about 30 seconds.
Season and finish the sauce. Sprinkle the flour around the outside of the well directly on the pasta, along with Cajun seasoning and the Worcestershire sauce. Then mix everything together thoroughly, and turn the griddle off.
Loosen with pasta water. Add about ¼-½ cup of the reserved pasta water, stirring to combine. There should be enough residual heat on the griddle to finish cooking everything through and let the sauce reduce and cling to the noodles.
Garnish and serve. Serve immediately, garnished with green onion tops and parsley if desired.