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Heat 1 tbsp of the oil in a flameproof dish over a medium heat. Add the onions and cook for 10 mins until softened, then stir in the flour and cook for a further 2 mins. Add the carrots, cauliflower, garlic and rosemary, and cook for 5 mins, stirring regularly, until they begin to soften.
Tip the tomatoes into the vegetables along with a can full of water. Cover with a lid and simmer for 10 mins, then remove the lid and cook for 10-15 mins more, until the sauce has thickened and the vegetables are cooked. Season, stir in the peas and cook for 1 min more.
Meanwhile, boil the potatoes for 10-15 mins until tender. Drain, then place back in the saucepan and mash. Stir through enough milk to reach a fairly soft consistency, then add the remaining olive oil and season.
Heat the grill. Spoon the hot vegetable mix into a pie dish, top with the mash and drag a fork lightly over the surface. Place under the grill for a few mins until the top is crisp golden brown.