Heat your oven to 180°C/Fan 160°C/Gas Mark 4. Remove the butter from the fridge. Scrub the potatoes. Place each potato in to the ‘bowl’ of a wooden spoon. Carefully slice into the potato at ½ cm intervals. The edges of the spoon will stop you slicing all the way through the potato. You can do it without the spoon, just be careful.
Peel and grate the garlic. Pop in a bowl with 1 tbsp olive oil with a little salt and pepper. Stir to mix. Pop the potatoes into a roasting tin. Pour the garlicky olive oil over the potatoes. Toss to coat. Put in oven. Set a timer for 30 mins.
Finely chop the tarragon. Grate or pare the zest of the lemon. Juice it and put the juice to one side. Pop the softened butter into a bowl. Add the tarragon, lemon zest and seasoning. Mix well. Shape into two fat logs. Wrap in cling film. Pop into the freezer to firm up.
Peel and slice the onions into 1cm thick rounds. Push out the middles to separate the rings. Place in the roasting tray with the potatoes after they have cooked for 30 mins. Drizzle with 1 tsp oil and season. Cook for 20 mins.
Brush the steaks with 1 tsp olive oil each and season. Heat a frying pan. Add the steaks and cook for 2 mins on each side for medium cooked steak. Cook for an extra 2 mins if you prefer your steak well done.
Remove one log of butter from the freezer. Slice into 2 rounds. Place on top of each steak for the final min of cooking.
Pop the rocket in a bowl. Mix the lemon juice with 1 tbsp olive oil. Drizzle over the rocket and toss to mix. Serve with the buttery steak, potatoes and onion rings.