Preheat the oven to 180°C, fan 160°C, gas 3.
Heat the oil in a heavy-based pan, add the onion and cook gently for about 5 mins. Add the garlic and cook gently for a few more minutes.
Add the chopped tomatoes, dried oregano and a pinch of sugar.
Put the frozen vegetables into an ovenproof dish and then pour the sauce over the top.
In a mixing bowl rub the butter into the flour using your fingers to make fine breadcrumbs. Add the oats and cheese and sprinkle this mixture over the top of the vegetables.
Put the crumble into the oven for 45 mins until the top is crunchy and golden and the tomato sauce is bubbling.
If you're serving to a baby, purée a little of the vegetable crumble and serve with fingers of bread.