Dissolve yeast and sugar in warm milk.
Add rest of ingredients and knead to a dough until smooth. (minimum 5 minutes)
Place dough in a slightly oiled bowl.
Covered, leave to rise in a warm place until double in size . (about 1 to 2 hours)
For the filling: Bring milk, butter and sugar to boil.
Add grounded poppy seeds, cinnamon and grated lemon peel. Boil while stirring for about 3 minutes. Leave to cool.
Peel and grate apples.
Add apples to poppy seed mix and leave to cool completely.
Devide dough in four equal pieces.
Roll out each piece to a circle.
Cut in 8 equal pieces (see picture).
Put 1 tsp filling on each piece.
Roll up to a croissant. While rolling pull out the middle tip a bit.
Place croissants on a baking tray with baking paper. Leave to rise for about 20 minutes. (covered)
Brush croissants with egg yolk-milk-mixture.
Bake in preheated oven at 180° C for about 14 minutes.
For Brioche Croissant without filling: Roll out the dough, brush in with melted butter and roll up to a croissant.