Place the chopped sweet potato in a medium to large pot. Top with enough cold water to cover and add a tsp of salt. Bring to the boil and cook for 10-15 minutes, until soft (but still holding their shape). Set aside to cool.
Finely slice the spring onions, keeping the white and green parts separate. Keep the green parts aside for serving.
Chop the dill, keeping two tablespoons aside for serving.
In a medium sized bowl, mix together the mayonnaise, yoghurt, mustard, lemon juice, gherkins, jalapeños, spring onion whites and dill. Season with salt and pepper to taste.
Place the cooled sweet potato chunks into a serving dish.
Gently mix in the mayonnaise mixture.
Top with the corn, spring onion greens and dill. Cover until ready to serve.