Preheat oven to 400F.
Place a large pot of salted water over high heat and bring to a boil. Add pasta to boiling water and cook just shy of al dente, according to package directions. Drain the cooked pasta and reserve.
In a medium-size mixing bowl, combine the ground sirloin with the grill seasoning, ketchup and yellow mustard. Grab about a tablespoon of meat in your hand and roll it into a small ball. Place the mini meatballs onto a baking sheet, drizzle with 2 tablespoons of EVOO and bake until cooked through, about 15 minutes. Turn off the oven and turn on the broiler.
While the mini meatballs are in the oven, heat a large saucepot over medium-high heat with 1 turn of the pan of EVOO, about 1 tablespoon, and the butter. Cook the red onion with some salt and pepper until softened, about 5 minutes. Sprinkle flour over top and cook for about 1 minute. Whisk in the milk, season with salt and pepper, and cook until thickened, about 5 minutes.
Remove sauce from heat and whisk in 1 cup of each cheese. Add the drained pasta to the cheese sauce along with the baked meatballs and toss to combine. Pour everything out into a casserole dish and sprinkle with the remaining cheddar and Swiss cheeses.
If serving immediately, toss the mac under the broiler until the top is golden brown and bubbly. Top with the chopped tomatoes and pickles, and serve.
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