Heat olive oil in a large frying pan and saute the onion and garlic until softened. Add the silverbeet, tomatoes and oregano. Continue to cook for about 5 minutes, stirring frequently,until the liquid has completely evaporated. Remove from heat and allow to cool.

Place the cooled mixture in a food processor or blender and add the basil, ricotta cheese, breadcrumbs, walnuts and nutmeg. Puree until smooth, then season with salt and pepper.

Melt butter in a saucepan. Stir in the flour and cook for 1 minute. Gradually stir in the milk, and heat until simmering and thickened. Stir in the cheese.

Preheat the oven to 190C. Grease a shallow ovenproof dish.

Using a teaspoon (or a piping bag with a wide nozzle) stuff each tube with filling and lay it in the dish. Pour the cheese sauce evenly over the cannelloni. Sprinkle with walnuts and bake for about 30 minutes, until bubbling and golden.