For the cauliflower lasagne sheets, process half of the cauliflower in a food processor until finely chopped.
Transfer to a microwave-safe bowl. Repeat with the remaining cauliflower.
Cover and microwave on high, stirring continuously for 10 minutes till tender.
Drain through a fine sieve, return to the bowl and add the egg and parmesan stir well to blend.
Season, and stir well to blend.
Preheat your oven to 180ºC (350ºF).
Line 2 large baking trays with baking paper.
Divide the cauliflower mixture between the prepared trays and use your fingertips to gently press each batch of cauliflower mixture to a rectangle of 22 x 30 cm (8 x 12 inches).
Cook for 20 minutes until the mixture has dried out.
Set aside to cool. Cut into 10 cm (4 inches) width lasagne sheets.
Using a large non-stick frying pan over high heat, heat the oil add the leek and reduce the heat to low and cook, stirring continuously for 4 minutes until soft.
Increase the heat to high add the chicken and cook, breaking them up with a wooden spoon for 5 minutes till cooked.
Add the mushrooms, stirring often for 5 minutes or until the mushrooms are golden.
Add the spinach, stirring occasionally for 4 minutes until the spinach is wilted.
Add the cream cheese, stirring for 2 minutes until the cream cheese has melted, stir through the tarragon and season, set aside.
Grease a square pan 19 x 24 cm ( 7 x 9 inches) baking dish.
Brush the base of the prepared dish with half of the tomato pasta sauce.
Place 2 pieces of lasagne over the pasta sauce.
Top with half of the chicken mixture and sprinkle with ⅓ cup mozzarella.
Repeat with another layer of cauliflower and the remaining chicken mixture.
Top with remainder lasagne.
Finish with pasta sauce and mozzarella.
Bake for 30 minutes or till golden, and set aside for 7 minutes to rest before serving.