Line a 1lb loaf tin with a double layer of clingfilm, making sure that the clingfilm overlaps the edges.
Bring a large saucepan of salted water to the boil and blanch the cauliflower florets for 1 minute. Drain and leave to cool to one side.
Heat 25ml of the olive oil in a large frying pan set over a moderate heat. Sweat the finely chopped white onion for a few minutes with a little seasoning, stirring frequently. Remove from the heat and set to one side.
Heat 25ml of the olive oil in a heavy-based saucepan set over a medium heat until hot. Sweat the red onion and red pepper for 3-4 minutes, stirring occasionally, then add the chopped tomatoes, dried oregano and some seasoning. Stir well and cook for a few more minutes until slightly thickened. Bring a large saucepan of salted water to the boil and blanche the carrot batons for 30 seconds. Drain and refresh in iced water.
Heat the rest of the olive oil in batches and wilt the spinach in batches, using fresh olive oil each time and a little seasoning. Transfer each batch of wilted spinach to a colander set over a bowl and press it against the colander to drain the excess water; you want the spinach mixture to be quite dry. Rub the inside of the loaf tin with any remaining olive oil then press most of the spinach into the base and sides in a layer roughly ½ cm thick.
Layer half of the blanched carrots across the base, making sure they are in an even layer. Spread out half of the tomato mixture on top, then top with a layer of the sweated white onion. Arrange the chives on top in an even row, then top them with the rest of the tomato mixture. Spread the blanched cauliflower florets on top, then layer the rest of the carrots on top. Cover the top of the carrots with the remaining spinach, making sure it evenly seals the carrots on top and at the sides.
Bring the overhanging clingfilm back over the top of the spinach to seal well. Pre-heat the oven to 120°C.
Bake the terrine for 1 hour then remove from the oven and leave to cool to one side. Place some weights on top of the terrine and chill overnight. When ready to serve, remove the weights and turn the terrine out onto a serving plate. Discard the clingfilm and use a sharp knife to cut into slices.