Heat the olive oil in a large saucepan set over a medium heat. Sweat the onion for 5 minutes, stirring occasionally, then add the peppers and courgette, stirring well. Reduce the heat, then add the seitan/tofu and canned chopped tomatoes and cover with a lid.
Cook gently for 6-8 minutes until heated through and the vegetables are soft. Adjust the seasoning as necessary, then stir in the pine nuts.
Spoon onto serving plates and sprinkle over some chopped tarragon. Garnish with tarragon stalks before serving.