Preheat the oven to 220 degrees C. Place the pumpkin and onions in a roasting pan and scatter over the garlic. Drizzle with the oil and season with pepper. Toss to coat the vegetables with the oil, then place the pan in the oven and roast for 15 minutes.
Remove the pan from the oven and add the leeks and asparagus. Toss gently to mix with the other vegetables, then return to the oven. Roast for a further 20 minutes or until all the vegetables are tender and beginning to brown.
Meanwhile, cook the pasta in a large saucepan of boiling water for 10–12 minutes, or according to the packet instructions.
Meanwhile, to make the sauce, measure 4 tablespoons milk into a jug, add the cornflour and stir to make a smooth paste. Heat the remaining milk in a saucepan until almost boiling. Stir the hot milk into the cornflour mixture, then return to the saucepan and heat gently, stirring, until the mixture boils and thickens. Simmer for 2 minutes. Remove the sauce from the heat and add two-thirds of the cheese and the mustard. Season with pepper.
Remove the vegetables from the oven. Drain the pasta well then tip over the vegetables and stir to combine. Stir in the sauce. Sprinkle the remaining cheese over the top. Return to the oven and bake for 10–15 minutes or until golden and bubbling. Serve hot.