Place a large pot of salted water over high heat and bring it to a boil.
Once the water boils, cook the cavatappi to al dente according to package directions. Once the pasta has finished cooking, drain it and return it to the pot it was cooked in.
While the pasta is cooking, place a large skillet over medium-high heat with 2 turns of the pan of EVOO, about 2 tablespoons. Add the onions to the pan and top them with a lid or piece of aluminum foil for 2-3 minutes, just to get them cooking. Remove the lid or aluminum foil and continue cooking the onions until tender and lightly caramelized, 8-10 minutes.
While the onions are caramelizing, melt the butter over medium-high heat in a medium-size pot. Sprinkle the flour over the melted butter and cook the mixture for about 1 minute. Whisk the milk into the butter-flour mixture slowly and bring up to a bubble to thicken. Remove the pot from the heat and stir in the shredded cheese. Season with salt and freshly ground black pepper and reserve.
To the skillet with the onions, add the beef stock and the roast beef. Cook to warm through.
Add the cheese sauce and the onion-roast beef mixture to the pot with the drained pasta and toss everything to combine.
Prepare your salad by tossing all the veggies together with the red wine vinegar, remaining 2 tablespoons EVOO, salt and freshly ground black pepper.
Portion the mac and cheese onto plates, topping each with a sprinkle of parsley, with some salad alongside.