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Adjust rack to top position (top and middle positions for 4 servings) and preheat oven to 450 degrees. Wash and dry produce.
Halve, peel, and thinly slice onion; mince a few slices until you have 2 TBSP (4 TBSP for 4). Dice tomato. Roughly chop cilantro. Quarter lemon. Halve, core, and thinly slice poblano into strips.
In a small bowl, combine minced onion, tomato, half the cilantro, and a squeeze of lemon juice. Season with salt and pepper.
In a separate small bowl, combine sour cream with a squeeze of lemon juice. Season with salt and pepper. Stir in water 1 tsp at a time until mixture reaches a drizzling consistency.