Pre-heat the oven to 180°C.
Cook the pasta in plenty of boiling, salted water until al dente (it should be very slightly under cooked whereby a couple more minutes would make it ready). Remove it from the heat and drain in a colander.
While the pasta is cooking, pierce and insert cloves all around the onion, put the oat milk in a saucepan, with the clove-studded onion, garlic cloves and bay leaf and any other left over veg trimmings and aromatics you want to use. Bring to a simmer and turn off the heat. Leave to stand.
While the milk is heating, in a separate pan heat the butter substitute or oil. When it's warm or melted, add the flour. Cook for a few minutes on a low heat, stirring regularly until sandy in texture. Remove from the heat and set aside.
Strain the oat milk through a fine sieve into a jug, discarding the vegetables and aromatics. Add the cooked flour and oil or butter mix to a clean, heavy based saucepan and place over a low heat.
Add a little of the milk, stirring carefully at first so as not to splash, and then vigorously to form a thick, smooth paste. Repeat this process with a little more milk, again stirring carefully and then vigorously. Keep repeating this, adding a little milk at a time and stirring until all the milk is used and you have a smooth sauce. Add the mustard powder and stir well to combine.
Bring it to a simmer and continue to cook gently for a couple of minutes, stirring. Add the reserved pasta water, grated vegan cheese and nutritional yeast. Keep stirring until the cheese is melted and the sauce is smooth again. Combine the sauce with the rigatoni and then pour into the baking dish.
Combine the oil and the breadcrumbs and stir well until evenly coated. Top the pasta mix with the breadcrumb and oil mix.
Bake in the oven for twenty minutes – serve immediately.