Melt butter in a saucepan, add onion, cook for 2-3 minutes, or until soft, seasoning to taste.
Combine onion, carrot, apple and celery pulp, cottage cheese, mint and egg in a bowl. Mix well.
Cut filo sheets in half, place 3 sheets on bench, cover remaining pastry with greaseproof paper, then a damp cloth to prevent drying. Brush 1 sheet of pastry with extra butter, sprinkle with Parmesan cheese, top with another sheet of pastry, brush with butter, sprinkle with more cheese. Repeat with last sheet of pastry.
Place tablespoons of carrot pulp mixture on one end of pastry, fold in sides and roll up like a swiss roll. Repeat with remaining pastry and pulp mixture. Place on a greased oven tray and bake at 400°F for 20-25 minutes or until golden.