Heat the oil in a small saucepan. Add the onions and cook over a low heat for a few minutes until soft but not brown.
Add the curry powder and fry for 2-3 minutes to help cook off the powdery taste.
Add the tomato purée, lemon juice, mango chutney and 150ml water. Simmer for about 20 minutes, stirring occasionally, until reduced to a thick sauce.
Pop into a mini blender or blend with a hand-held blender until really smooth. Spoon into a bowl and leave to go cold. Add the yogurt and mayonnaise and mix together. Spoon a little (just under a third) into a little bowl for the little ones.
Add the adults’ chicken to the remaining dressing and mix. For adults, scatter lettuce and grapes on plates, top with the chicken and dressing mixture.
Tip: for toddlers aged 18 months plus, serve pieces of chicken and some crunchy salad and grapes and then dollop a small spoonful of the dressing on the side, this way the toddler can always dip the chicken into the dressing if they're keen to try it.