Peel and slice the garlic. Roughly chop the sage leaves. Slice the mushrooms into 1cm thick slices.
Melt half the butter in a large frying pan with 2 tsp olive oil. Add the garlic, sage and mushrooms with a pinch of salt. Fry over a medium heat, stirring often, for around 10 mins till the mushrooms are soft and golden. Take off the heat and pop on a lid to keep warm.
While the mushrooms cook, scrub the potatoes and halve any that are on the large side. Put in a pan of cold water. Cover, bring to the boil and simmer for 6-8 mins till they start to feel tender when you press a fork into them.
While the potatoes cook, trim the ends of the French beans. Add them to the potatoes and simmer for 2 mins. Drain the veg. Return them to the pan and toss through the remaining butter, a good pinch of sea salt and a squeeze of lemon juice. Pop on a lid to keep them warm.
Squeeze a little lemon juice over the mushroom mixture, then tip onto a warm plate and set aside.
Put the frying pan back over a high heat and add 1 tsp oil. Season the gammon steaks with plenty of black pepper. Fry for 6-8 mins, turning once, till golden and cooked through. Return the mushroom mixture to the pan and cook together for 1-2 mins to heat through.
Divide the bean and potato mixture between two plates and serve with the gammon and herby mushrooms.