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Place tomatoes in a sieve; toss with 1/4 teaspoon sea salt. Set aside to drain.
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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.
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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.
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Position rack in top third of oven. Heat broiler to high.
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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.
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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.