Coat glasses with oil. Preheat oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer). Cream butter, sugar and lemon juice with the whisks of the mixer. Stir in the eggs one by one.
Stir in flour, baking powder and starch.
Peel the banana, halve it lengthwise and cut into thin slices. Coarsely chop the walnuts. Fold banana and nuts into the dough. Fill into the glasses in the same way. Bake in the oven for about 35 minutes
Simmer the lid and preserving rings in boiling water for 2 minutes. Drain. Remove the cakes. Close jars immediately with preserving rings, lids and clips. Let cool down