Preheat the oven to 200°C. Turn the cauliflower upside down with the stalk facing up. Carefully cut through the stalk at one end, then slice 2 large steaks out of the cauliflower head about 2cm-thick. Slice any larger florets into steaks.
In 2 batches, heat 1 tbs oil in a large frypan over medium-high heat. Season the steaks with salt and pepper on both sides. Cook the steaks for 5 minutes, turning, until golden. Transfer to a large tray lined with baking paper with any remaining florets. Roast the steaks in the oven for 10-12 minutes or until golden and cooked through.
Meanwhile, heat the butter and remaining oil together in a large frying pan over medium heat. Add onion. Cook, stirring occasionally, for 5 minutes, or until soft.
Add mushrooms. Cook, stirring occasionally for 5 minutes, or until softened. Sprinkle over flour. Cook, stirring for 1 minute.
Add stock, stirring constantly. Bring to boil. Simmer, uncovered over a medium-low heat, for 5 minutes. Stir in cream and mustard.
Simmer, stirring occasionally, for a further 8-10 minutes, or until thickened slightly. Stir in parsley. Season.
Serve the cauliflower with the mushroom sauce.