Use your fingers to rub the lemon zest into the sugar to flavor it. Beat the egg and sugar until creamy. Stir in the rest of the batter ingredients - yogurt , oil and vanilla. Now slowly add the flour mixture and mix in briefly.
For the streusel place the flour, sugar and cinnamon (optional) in a bowl with a pinch of salt and mix together. Melt the butter slowly on the stove over low heat, lightly brown it if you have the patience and allow to cool slightly. Add the butter and mix with a fork or hand mixer until crumbles form. If they are too dry add a little more butter.
Core the apple, dice and mix with the cinnamon and starch. Heat briefly in a small saucepan with a small splash of water and lemon juice while stirring. This step makes for juicy apple chunks.
Line a muffin tin with paper cups and pour the muffin batter into the cups. These should only be half full. Then top with the apple pieces and press lightly into the dough. Finally, sprinkle the crumble topping generously over the muffins.