Heat your oven to 200°C/Fan 180°C/Gas 6. Add 1 tbsp oil to a roasting tin. Heat it in the oven for
10 mins while you prepare the celeriac.
Peel the celeriac, slicing off any knobbly roots. Cut the celeriac into 2cm slices then into 2cm thick chips. Tumble carefully into the roasting tin. Cook for 40 mins, tossing halfway through.
Meanwhile, grate half the Parmesan into a bowl. Peel the horseradish and grate that in too. Peel and grate or crush the garlic. Add to the bowl with 1 tsp Dijon mustard. Smoosh together, forming a thick paste. Set aside.
Peel and thinly slice the onions. Heat ½ tbsp oil in a small pan and add the onion. Gently fry for
10 mins till soft. Sprinkle in 1 tbsp sugar and stir to dissolve. Pour in 1 tbsp balsamic vinegar. Allow to bubble up, then turn the heat right down. Cover and leave to gently cook while you cook your steaks.
Rub each steak with ½ tsp oil and some salt and pepper. Heat a griddle or frying pan till smoking hot. Fry the steaks for 1-2 mins on each side, depending on how well done you like your steak.
Heat your grill to its highest setting. Pop the steaks onto a flat baking tray. Dollop the Parmesan and horseradish mix onto the steaks and smooth all over with the back of a spoon.
Slide the steaks under the grill for 2 mins till bubbling and golden.
Serve the horseradish rarebit steaks with a spoonful of the sticky onion jam, celeriac chips
and a handful of rocket.