Crumble the vegetable stock cube into a heatproof jug. Tip in the porcini mushrooms. Pour in 450ml boiling water and stir to dissolve the stock cube. Set aside to soak.
Peel and finely slice the shallots. Thickly slice the chestnut mushrooms, halving or quartering any small mushrooms. Peel and crush the garlic. Snap the woody ends from the asparagus so you just have the tender tips. Finely grate the zest from the lemon.
Put a deep frying pan or wok over a medium heat. Warm for 1 min, then add 2 tsp oil to the pan and the shallots. Season with a pinch of salt and pepper. Fry, stirring, for 5 mins till the shallots have softened and started to brown and turn golden.
Add the mushrooms to the pan and fry for 5 mins, stirring often, till the mushrooms are juicy and soft.
Stir in the garlic, lemon zest and asparagus tips. Tip in the gnocchi and stir to mix well, then pour in the porcini stock with the porcini mushrooms. Pop on a lid. Simmer for 5 mins till the veg are tender and the gnocchi are cooked.
While the veg and gnocchi simmer, roughly chop the tarragon leaves.
Add a squeeze of lemon juice to the pan and taste the dish. Season with a squeeze more lemon juice, salt or pepper. Ladle the gnocchi, veg and broth into warm bowls and serve garnished with the chopped tarragon.