Put a large pan of salted water on to boil. Scrub the potatoes and chop them into equal-sized chunks. When the water is boiling, add the potatoes. Simmer for 15 mins till the potatoes are tender when pressed with a fork.
While the potatoes simmer, open up the pea pods and scoop out the peas (see our tip below on what to with the pea pods). Halve the pepper and scoop out the seeds. Roughly chop it. Finely grate the zest from the lemon and set aside. Chop the lemon into wedges. Finely slice the mint leaves.
Drain the potatoes and set aside.
Heat your grill to high. Put a large frying pan or wok onto a medium heat. Let it warm for 1 min, then add 1/2 tbsp olive oil and the peppers. Stir and fry for 5 mins till the pepper is browned all over. Lift out of the pan and set aside.
Add another 1 tbsp oil to the pan. Tip in the potatoes. Season with salt and pepper. Keep the heat set to medium-low. Cook for 10 mins, stirring now and then, till the potatoes are browned all over. Lightly squash the potatoes with a fork or potato masher as they fry.
While the potatoes fry, line a baking tray with baking paper. Place the hake fillets on it, skin-side up. Season with a pinch of salt and pepper. Grill for 10-12 mins till the skin is crisp and the fish is white and flakes when pressed with a fork.
When the potatoes have browned a little, add the red pepper back to the pan and tip in the peas. Add the lemon zest, most of the mint and the red wine vinegar. Cook and stir for 3 mins till the potatoes are browned and the peas are tender. Taste and add a little more salt and pepper if you think it needs it.
Serve the grilled hake fillets with the new potato smash, scattered with the last of the mint, and with lemon wedges for squeezing.