Preheat the oven to 190 degrees C. First make the batter. Sift the flour into a bowl, add the egg and a pinch of salt, then gradually whisk in the milk to form a smooth batter. (This can be done in a blender or food processor.) Transfer to a jug.
Heat a frying pan, preferably non-stick, and brush lightly with a little oil. Pour one-eighth of the batter into the pan and quickly swirl to coat the bottom. Cook for 1–2 minutes until set, then flip over and cook the other side. Remove and place on paper towel. Repeat to make eight pancakes, greasing the pan with oil as necessary. Stack the pancakes, interleaved with paper towel or baking paper. (The pancakes can be frozen.)
To make the filling, cook the peas and broccoli in a saucepan of boiling water, or in the microwave, following the packet instructions. Drain, then roughly chop the broccoli. Meanwhile, melt the butter in a saucepan, add the onion and cook gently for 3–4 minutes until softened. Add the flour and cook, stirring, for 1 minute, then gradually stir in the milk. Bring to the boil, still stirring, then simmer for 2–3 minutes. Stir in the chicken, peas, broccoli and tarragon and season.
Spoon some filling onto the centre of each pancake, spreading it out a little, then roll up to enclose the filling. Arrange the filled pancakes, in a single layer, in a lightly greased, shallow oblong ovenproof dish, such as a lasagne dish, then sprinkle evenly with the cheese. Bake for 25 minutes or until golden.