Peel, halve and core the apples, then cut into dice and toss in 1 tsp lemon juice to prevent browning. Heat the butter in a frying pan over a low heat and sauté the apples until just starting to soften. Add the rest of the lemon juice and cook until the apples are completely soft; do not allow them to brown.
Tip the apples into a blender and purée until smooth.