Steak Frites with Garlic Mayo

6-8oz (168-224g) beef steak of choice - always well marbled
1 large baking potato
2 tbsp olive oil
1/4 cup of light mayonnaise
1/4 small spring onion - finely diced
1/4 tsp mixed herbs
1/4 garlic clove - micro diced
Handful of salad leaves
Smear of your favourite mustard - wholegrain, dijon, French…
2-3 pinches of flakey salt