Coat the sweet potato wedges in the olive oil and season with a little salt and pepper (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.
Cook the peas according to the pack instructions. Drain then roughly mash with the avocado, 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.