Trim the tough ends off the fennel and pick off any frondy bits. Finely slice the fennel bulb. Peel and finely slice the onion. Peel and halve the shallots.
Warm 1 tbsp olive oil in a heavy-based frying pan. Add the fennel, onion and shallots. Season. Cook over a low heat for 10 mins till the veg are golden and soft. Stir often.
While the veg cook, crumble the stock cube into a heatproof jug. Add 400ml boiling water. Stir to dissolve the stock cube.
Pop the peas from their pods. Trim the French beans. Pick the tarragon leaves off the stalks. Roughly chop the leaves.
Heat your grill to high. Line a baking tray with baking paper. Brush it with ½ tbsp olive oil. Lay the hake on the greased paper, skin side up.
Grill the hake for 8-12 mins till the skin is crisp and the hake is cooked all the way through. The flesh should be opaque and flake easily when pressed with a fork.
Add the peas and green beans to the fennel. Pour in the hot stock. Simmer, without a lid, for 5 mins till the veg are just tender. Stir in the tarragon. Taste and adjust the seasoning.
Ladle the veg into shallow bowls or plates. Top with the crispy skinned hake fillets and serve straight away.