To make the dressing, in a blender, combine the sour cream, buttermilk, garlic, chives, cilantro, and salt. Using a spoon, scoop the flesh from each avocado half into the blender. Pulse until mixture is smooth but still thick. Pour the dressing into a bowl, cover, and refrigerate until needed; it will keep for up to 2 days.

To make the spice rub, whisk together spices and seasonings for the rub. Working a few at a time, toss the shrimp in the rub to coat and set aside.

Heat the canola oil in a large skillet over medium-high heat. When the oil shimmers, carefully add the shrimp and cook for 1-2 minutes, until they start to turn pink. Flip the shrimp and cook until just done, 1-2 additional minutes. Transfer to paper towels to drain.

To assemble, toss the salad greens, carrot, and cucumber. Stir the dressing and pour about ½ cup of dressing over the salad; toss to coat. Divide among 4 chilled salad plates. Slice the avocado halves into ¼ inch slices and remove the peel; use alternating pieces of avocado, tomatoes, and shrimp to evenly top each salad. Drizzle the remaining dressing over each salad and season to taste with salt and pepper.