Heat your oven to 180°C/Fan 160°C/Gas 4. Pour 700ml boiling water into a jug. Crumble in the stock cube and add the dried porcini. Leave to soak for 10 mins.
Peel and finely dice the onion. Peel and slice 1 garlic clove. Heat 1 tbsp oil in an ovenproof pan or dish (see our tip). Add the onion and cook for 5-6 mins till soft. Add the garlic and cook for a further 2 mins.
Slice the portobello mushrooms and add them to the pan. Cook for 3 mins till just softened.
Pour in the risotto rice and stir to coat in the oil. Add the stock with the soaked porcini. Stir to combine. Cover with a lid and slide into the oven. Cook for 40 mins till the liquid is absorbed. You won’t need to stir the risotto.
While the risotto is cooking, strip the leaves from the tarragon and finely chop them. Zest and juice the lemon. Peel and grate or crush the remaining garlic clove. Place in a small bowl with 1 tbsp olive oil. Season. Stir well.
Remove the risotto from the oven. Stir and taste. Season if you think it needs it, but keep in mind that the tarragon gremolata is packed with flavour.
Serve the risotto with the lamb's lettuce on the side, being sure to drizzle both generously with the zingy gremolata.