Choose tomatoes that are ripe but firm. Scoop up some of the dressing with the tomatoes and shallot mixture when serving.
Combine ⅛ teaspoon salt, cumin, paprika, sugar, and pepper in small bowl. Toss shallot, cilantro, oil, and lime juice together in second small bowl.
Whisk sour cream, mayonnaise, garlic, lime zest, and remaining ¼ teaspoon salt in bowl until combined. Spread dressing evenly in large circle over bottom of large plate.
Arrange tomatoes, cut sides up, on top of dressing. Sprinkle spice mixture evenly over tomatoes. Garnish with shallot mixture and serve.
