Heat your oven to 180°C/Fan 160°C/Gas 4. Boil the kettle and pour 700ml hot water into a jug. Crumble in the stock cube and add the dried porcini. Leave to soak for 10 mins.
Peel and finely dice your onion. Peel and slice 1 garlic clove. Heat 1 tbsp oil in an ovenproof pan or dish. Add the onion and cook for 5-6 mins till soft. Add the garlic and cook for 2 mins.
Slice the Portobello mushrooms and add 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. Cover with a lid and place in the oven. Cook for 40 mins till the liquid has been 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 into a small bowl with 1 tbsp olive oil. Season. Stir well.
Remove the risotto from the oven. Stir and season to taste. Add more if you think it needs it, but the tarragon gremolata is very flavourful.
Serve the risotto with the gremolata spooned over the top and drizzled on the lamb’s lettuce.