Preheat the oven to 180C/ 350F/ Gas Mark 4. Boil the pasta until al dente, then drain and refresh in cold water to stop it from over cooking. Melt the butter in a saucepan and sauté the onion until soft. Stir the cornflour into the water until dissolved, then add to the pan along with the tomato soup. Bring to the boil, add the herbs then cook gently stirring for five minutes. Mix in the flaked tuna and heat through. Season to taste. For the sauce, melt the butter in a saucepan and sauté the onion until transparent. Add the flour and continue stirring the mixture all the time. Gradually pour in the milk and keep stirring until the sauce is thickened and smooth. Add the chopped spinach and stir until wilted. Remove from the heat and stir in the grated cheese. Grease a serving dish and add the tuna and tomato mixed with the pasta and then the spinach sauce. Sprinkle with some grated Cheddar Cheese. Bake for 25 minutes until golden.