Preheat the oven to 200C, fan-forced. Bring a small saucepan of water to the boil for the vegetables. Cut the cauliflower stem into 1cm chunks, then cut the head into small florets. Cut the onion into thin wedges. Finely chop the parsley leaves and stems. Crush or finely chop the garlic. Finely grate the parmesan.
Step 2 — Make crumb mixture
Cook the cauliflower and onion in the pan of boiling water for 4 mins. Drain. Reserve the pan. Meanwhile, put the breadcrumbs, parsley, half the parmesan and 2 tsp olive oil in a bowl and massage until well combined. Season with salt and pepper. For the sauce, combine the garlic, 2 tbs water and 1 tsp dijon mustard in a separate bowl.
Step 3 — Make bechamel
Melt 20g butter in the reserved pan over medium-high heat. Add 1 tbs flour and cook, stirring, for 1 min or until golden. Gradually add 125ml (½ cup) milk and 80ml (⅓ cup) water, stirring constantly to prevent lumps forming. Bring to a simmer and cook, stirring, for 1 min or until thickened. Stir in the remaining parmesan. Taste, then season with salt and pepper.
Step 4 — Bake gratin
Put the cauliflower and onion in a 500ml (2 cup) baking dish. Pour over the bechamel and scatter with the crumb mixture. Bake for 20 mins or until golden.
Step 5 — Cook steaks
Once the gratin has baked for 10 mins, combine the steaks, country seasoning and 2 tsp olive oil in a bowl. Heat a medium frypan over high heat until hot (see cooking tip). Reduce the heat to medium-high and cook the steaks, turning regularly, for 4-6 mins for medium-rare, or until cooked to your liking. Remove from the pan and rest for 3 mins.
Step 6 — Make sauce and toss salad
Return the same pan to medium heat. Add the mustard mixture and cook, stirring, for 20 secs or until slightly thickened. Add 10g butter and stir until melted, then season with salt and pepper. Combine the rocket and vinaigrette in a bowl. Divide the steaks and gratin among plates. Drizzle the steaks with the mustard sauce and serve with the rocket salad.