1. Heat a charcoal grill to medium-hot coals or a gas grill to medium-high heat.
2. In a large bowl, whisk sour cream, oil, lime juice, chili powder and sugar. Sprinkle with 1/4 teaspoon of the salt and 1/8 teaspoon of the pepper. Refrigerate until ready to serve.
3. Brush tomato and scallions with more olive oil using enough to lightly coat vegetables to keep them from sticking. Season with remaining 1/4 teaspoon salt and 1/8 teaspoon pepper. Grill for 3 to 5 minutes, until beginning to char, then carefully flip and cook for a few minutes. Remove vegetables. When cool enough to handle, chop scallions into bite-size pieces and cut tomato slices in half, saving their juices if possible. (Can be made ahead and refrigerated for 1 hour.)
4. Grill sausages until heated through, about 5 to 7 minutes. Cut into coins. Put lettuce, sausage, avocado, half of the cilantro, the scallions and juice from tomatoes into the bowl with the dressing. Gently toss. Taste and adjust seasoning as needed. Divide chips onto 4 plates. Top with salad and tomato slices. Garnish with remaining cilantro.