Preheat the oven to 400°F (200°C) and grease an ovenproof serving dish.

Heat the oil in a pan over high heat.

Sauté the onion, chicken, and ground beef for 4 minutes, stirring frequently.

Add the sherry and season with salt and pepper to taste.

Stir in the parsley and breadcrumbs to bind the mixture.

Fill each cannelloni tube with the prepared meat mixture.

To prepare the bechamel sauce, melt the butter in a saucepan over medium heat.

Add the flour and cook for 2 minutes, stirring constantly to form a roux.

Gradually pour in the milk, stirring constantly to avoid lumps, and bring to a boil.

Reduce heat and simmer until the sauce thickens, stirring continuously.

Add half of the grated cheddar cheese and stir until melted and smooth.

Season the bechamel sauce with salt, black pepper, and a pinch of nutmeg.

Spread half of the bechamel sauce evenly in the greased serving dish.

Arrange the filled cannelloni tubes on top of the sauce.

Sprinkle the diced tomatoes and ham evenly over the cannelloni.

Cover the cannelloni with the remaining bechamel sauce.

Sprinkle the remaining cheddar cheese over the top of the sauce.

Bake in the preheated oven for 40 minutes, or until golden brown and bubbly.

Garnish with fresh herbs before serving.