Place tomatoes in a sieve; toss with 1/4 teaspoon sea salt. Set aside to drain.
Bring large pot of water with 1 teaspoon sea salt to a boil. Cook macaroni just to al dente – do not overcook. Drain and set aside.
Heat olive oil in a wide, deep, broiler-proof skillet over medium-high heat. Add onions, paprika and cumin; sauté 3 minutes, stirring often. Add garlic and sauté additional 1 minute. Add ground beef; sprinkle with remaining 1 teaspoon sea salt. Cook until meat is no longer pink, about 4 minutes. Turn off heat and fold in 1 cup reserved tomatoes, arugula and cooked macaroni. Set aside.
Position rack in top third of oven. Heat broiler to high.
Melt butter in large saucepan over medium heat. Whisk in flour; gradually whisk in milk, allowing mixture to thicken before adding additional. When all the milk is added, cook mixture, stirring constantly, over medium-low heat until it is thick enough to coat back of a wooden spoon, 5 to 7 minutes. Remove from heat; fold in cheddar, stirring until smooth.
Fold cheese sauce into macaroni mixture. Sprinkle with parmesan and dot with remaining tomatoes. Place under hot broiler; broil until top is slightly browned and mixture is bubbling, 3 to 5 minutes. Serve immediately.