Place half of the potatoes in a large baking dish. Sprinkle the chopped shallots over the top, along with the butter broken up into knobs. Season with salt and freshly ground black pepper. Add the silverbeet, leek, green beans and cauliflower.
To make the cheese sauce; heat the butter over medium heat in a medium sized pot. Add the flour and stir with the butter to combine well. Slowly add the milk to the pot, stirring constantly to ensure a smooth mixture. Reduce heat to low and add half of the cheese. Stir until cheese has melted, remove from heat and pour 3/4 of the mixture over the vegetables.
Layer the remaining potatoes on top of the cheese sauce and vegetables. Cover with the remaining sauce. Sprinkle remaining cheese over top. Bake in the preheated oven for 1 hour, or until vegetables are tender and cheese is crisp.