Heat the oil in a large pan. Add the onion and carrots and cook for 5 minutes until softened. Add 1 tsp tarragon and 20g flour for the final 2 minutes.
Stir in the passata, then cook for 4-5 minutes. Stir through half the soft cheese, the chicken breast pieces and the spinach. Cook for 2-3 minutes until the spinach has wilted.
Meanwhile, pour the milk into a small pan and set over a medium-low heat. Mix together the remaining flour with 3 tbsp water to form a paste. Just before the milk comes to a simmer, whisk in the paste, whisking continuously for 4-5 minutes until the sauce has thickened. Remove from the heat and stir in the remaining soft cheese.
Preheat the oven to 200°C/gas mark 6. Spoon 1⁄3 of the chicken mixture into a 1½-litre baking dish, then top with lasagne sheets and a layer of white sauce. Continue layering the chicken mixture, sauce and lasagne sheets, finishing with a lasagne sheet topped with white sauce. Sprinkle over the grated cheese and remaining tarragon. Bake for 30 minutes, until golden. Serve with the rocket leaves.