Preheat your oven to 200°C/Fan 180°C/Gas 6. Pour the barley into a pan and add a pinch of salt. Cover the barley with 200ml boiling water. Set the pan on the hob and bring to the boil, then reduce to a simmer. Cover and cook for 25 mins, till tender.
Meanwhile, trim the root and stalk away from the red Florence onion. Slice it into bite-sized chunks. Trim the root and top 3cm off the leeks, and rinse them under a cold tap. Slice the leeks into 1cm-thick rounds. Pull the leaves and silks off the sweetcorn cobs. Scatter the chopped onion and leeks into a large roasting tin. Nestle in the whole sweetcorn cobs. Crumble over large chunks of feta. Drizzle with 1 tsp olive oil and a pinch of salt and pepper. Fold together, then slide into the oven for 20 mins.
When the barley is cooked, drain if necessary and tip into a bowl. Peel and grate in the garlic clove. Grate in the lemon zest and squeeze in the juice. Pick the tarragon leaves and add to the barley with 1 tsp olive oil.
Remove the roasted veg from the oven, removing the corn cobs from the tin and allowing them to cool slightly. Carefully stand the cobs upright on a chopping board and run a knife down the sides to slice off the kernels. Tip the kernels into the barley, along with the onion, leeks and feta. Add the rocket leaves and stir to mix. Have a taste and add more salt and pepper if you like.
Divide the barley and veg between a couple of plates, and dollop over the yogurt to serve.