Chop or blitz the cauliflower into rice-sized pieces. Cover loosely and microwave for 5 mins, or until tender, stirring halfway. Pour the milk into a saucepan with the plain flour and 1tbsp butter. Whisk together and slowly bring to the boil, mixing all the time, until smooth and thick. Mix in the cooked cauliflower, cheddar and parmesan, then season. Leave to cool in the pan. Once cold, transfer to a piping bag or food bag and chill.
Heat oven to 200C/180C fan/gas 6. Line a baking sheet with baking parchment. Unfold the pastry sheets and slice into three pieces widthways, cutting through all the sheets at once. Take out three strips and keep the rest covered.
Melt the remaining butter, then use to lightly brush the pastry strips. Fold the strips in half lengthways, then brush again with the melted butter. Pipe or spread 1 tbsp of the cauliflower mixture onto the buttered pastry strips. Fold the short ends in, then loosely roll to form a cigar. Brush the end with a little butter to help seal in place. Place on the baking sheet, spacing them a little apart, then repeat with the remaining pastry strips. Brush the tops with butter, then sprinkle with the za’atar and sesame seeds. Bake for 15-18 mins until golden and crisp. Serve warm.