Measure the sour cream, salsa, mayonnaise, lime juice, taco seasoning, garlic powder, and ground cumin. Fresh lime juice provides a brighter flavor than bottled juice, while your favorite salsa determines much of the dressing's heat and texture.
Add the sour cream and mayonnaise to a small mixing bowl. Whisk until smooth and evenly combined. This creamy base gives the dressing its thick consistency and helps it cling to lettuce and other salad ingredients.
Stir the salsa into the creamy mixture until the color is uniform. Use a finely chopped salsa for a smoother dressing or a chunky variety for more texture. If the salsa contains a lot of liquid, add it gradually to maintain the consistency you prefer.
Add the taco seasoning, garlic powder, and ground cumin. Whisk thoroughly to distribute the spices throughout the mixture. Taste the dressing before adding salt because prepared taco seasoning and salsa may already contain enough sodium.
Pour in the fresh lime juice and whisk again. The citrus adds acidity that balances the richness of the sour cream and mayonnaise. Add a little more lime juice if you prefer a sharper, tangier flavor.
Check the texture of the dressing. For a thick dip-style consistency, leave it as prepared. For a thinner salad dressing, whisk in water one teaspoon at a time until it is easy to drizzle. You can also add a small spoonful of salsa for extra flavor.
Taste the dressing and adjust the seasoning as needed. Add more taco seasoning for a stronger savory flavor, extra lime juice for tanginess, or a little hot sauce for additional heat. Stir well after each adjustment.
Pour the dressing over a freshly assembled taco salad or transfer it to an airtight container. You can serve it immediately or refrigerate it for 15–30 minutes to allow the flavors to blend. Stir before serving if the ingredients have separated slightly.