Combine the tomatoes, scallion greens, lemon juice and a pinch of salt in a medium bowl and toss to coat.
Add the oil to a large skillet and heat over medium-high heat. Add the tomato paste, Cajun seasoning, scallion whites, celery and garlic. Cook, stirring frequently, until the paste turns brick red and the scallions and celery just begin to soften, about 4 minutes. Add the quinoa and stir to coat in the tomato paste mixture. Pour in 2 cups water and sprinkle with 1/2 teaspoon salt and several grinds of pepper. Bring to a high simmer and cook until most of the liquid has evaporated and the quinoa is tender, 12 to 15 minutes.
Adjust the heat to medium-low, add the shrimp, sprinkle with salt and pepper, cover and cook until the shrimp is firm and cooked through, about 5 minutes. Remove from the heat and scatter with the cherry tomato salad.
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