Make sure the meat is at room temperature before you start cooking.
Preheat oven to 170oC. To make the celeriac gratin, melt the butter in a heavy-bottomed saucepan. Stirring constantly, mix in the flour and cook until the paste bubbles a bit, about 2 minutes – but don't let it brown.
Add the hot milk, continuing to stir as the sauce thickens. Bring it to a simmer, add cheese and salt and pepper to taste. Lower the heat and cook, stirring continuously for 2 to 3 minutes. Remove from the heat.
Grease a deep baking tray and place a layer of celeriac on the base, followed by a layer of onions and some bechamel sauce. Repeat process until everything has been used. Sprinkle top with panko crumbs and a drizzle of olive oil.
Place in heated oven for 30-40 minutes. Increase heat at the end if you want to develop more colour on top.
While this is cooking, combine all your dressing ingredients then place this into a bowl with the salad ingredients. Slightly massage the salad to soften the kale.
Prepare the now room temperature steak by trimming, patting dry with paper towel and rubbing with some oil and season both sides with generous amounts of salt.
Heat a heavy-based frypan on med-high heat and cook the steak for 4-5 minutes on each side to develop a nice crust (depending on the thickness of your rib eye cooking time will vary).
For a large rib eyes, remove from fridge at least two hours before to allow sufficient time to get to room temperature.
Use a cooking thermometer to check the internal temp for your desired outcome as the most accurate way of cooking.