Peel the potatoes, carrots and parsnips and dice or julienne. Remove seeds from bell peppers and chop into 1-inch pieces. Peel the red onions and slice into wedges.
Put all the vegetables in a bowl. Drizzle over the oil and season with rosemary, salt and black pepper. Pour the vegetables into the Omnia pan.
Cover and bake on medium heat for about 25 minutes. Check the vegetables and turn gently now and then.