Preheat the oven to 375 and grease your chosen casserole dish.  Put the cauliflower, onion and garlic into a steamer basket over water, then cover and steam for about 15 minutes until the vegetables are tender.  Transfer all of the steamed vegetables to your food processor and pulse until smooth.

Meanwhile cook the penne in boiling, salted water – I’d say al dente, but you want it even less than that – keep it quite firm as it will soften way more in the oven.  Drain the pasta and return it to the pot.

To your food processor, add 1 cup of the milk, the mustard, salt and pepper and puree until smooth, then transfer it all to a large pot.  Stir in the remaining milk and thyme and heat gently until the mixture is hot, but not boiling, then remove the pot from the heat and add in 3/4’s of all of the cheese.  Stir until the cheese has melted, add the sauce to the pasta (or vice versa depending on pot sizes) then transfer it all to your casserole, and sprinkle with the remaining cheese.  Bake for 30-40 minutes until heated through and your pasta is tender.  This (and the leftovers) was lovely with a large salad.