For the dressing: Combine the buttermilk, mayonnaise, sour cream, dill, garlic powder, vinegar and a pinch of salt and pepper in a bowl. Stir to combine. Taste and adjust the seasoning as needed. Transfer to a ramekin.
For the salad: Place the ramekin in the center of a platter. Arrange the lettuce around the dressing. Layer the tomatoes, cucumbers and bell peppers on top of the lettuce and serve.