This topping can be made ahead and kept refrigerated for up to 5 days. Serve cold, or warm briefly in the microwave or a warm skillet before serving.
1 Cook the onion, garlic and spices: In a large, deep skillet over medium heat, heat the olive oil. Add the onion, garlic, chili powder, cumin, oregano, salt and a few grinds of fresh black pepper.
Cook, stirring occasionally for 5 minutes, or until the onions soften.
2 Add the ground turkey and continue cooking. Break up the meat into small bits as it cooks using a stiff spatula. Cook for about 4 minutes, or until the turkey is no longer pink.
3 Stir in the diced tomatoes, black beans and corn. Reduce the heat to medium-low and cook for about 10 more minutes to allow the flavors to mingle and the mixture to thicken. If the mixture seems dry, add water 2 tablespoons at a time.
Remove the skillet from the heat. Taste for seasoning and add more salt, if you like. Cool briefly.
4 Dress the greens: In a bowl large enough to hold the lettuce, whisk together the vinegar, 1/4 teaspoon salt and a few grinds of fresh black pepper. Gradually whisk in the olive oil until the dressing looks combined.
Add the lettuce to the bowl. Toss to coat it with the dressing. Taste for seasoning and add more salt and pepper, if you like.
5 Assemble the salad: Divide the lettuce among 4 to 6 plates. Divide the filling between the plates, and garnish with the tomatoes, onions, avocado, radishes, shredded cheese and cilantro. Top each salad with spoonful of sour cream and a lime wedge, and serve.