Preheat the oven to gas 6, 200°C, fan 180°C. Coat the sweet potato wedges in the olive oil and season generously (it's easiest to do this by placing everything in a small plastic bag and giving it a shake). Spread out in a single layer in a roasting tin and cook for 20 minutes, turning halfway through.
Place the butter in a bowl and use a fork to mash in the crushed garlic and parsley. Season. Spread the garlic butter over the hake and then place the fillet on a lightly greased baking tray. Bake for 5 minutes.
Transfer the fish to a warm serving plate and cover with foil, then tip the peas onto the hot baking tray. Warm in the oven for 5 minutes.
Transfer the warmed peas to a bowl. Peel and chop the avocado and roughly mash with the peas, adding a squeeze of lemon juice and a little seasoning as you go. Serve the fillet of hake with the sweet potato wedges and crushed peas.
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