For the rhubarb-lilac compote, first make the syrup. To do this, sort out the lilac blossoms, checking for insects if necessary. Cut off thick flower stalks. Wash the lemons and cut them into slices.
Boil the sugar together with 1.5 l water and the ascorbic acid (optional, prolongs shelf life) in a pot until the sugar has dissolved. Then let cool a bit and put in the lilac flowers and lemon slices. Refrigerate for about 5 days. Then drain the syrup, boil it once and fill it into clean or boiled twist-off bottles while it is still hot. For the compote, wash the rhubarb, clean it, pull the strings if necessary and cut it into pieces about 1 cm wide. Place in a saucepan with the syrup and bring to a boil while stirring.
Mix the custard powder with 1-2 tablespoons cold water until smooth, stir into the rhubarb and let the compote thicken slightly until creamy. Pour into glasses or small bowls and let cool. Serve the rhubarb-lilac compote drizzled with yogurt or whipped cream.