Heat the olive oil and butter in a large pan over medium heat. Add the diced onion and cook for 5 minutes until soft.
Add the garlic and pearl barley. Stir for 1 minute to coat the grains in the oil.
Pour in the vegetable stock and dried thyme. Bring to the boil, then reduce to a simmer.
Cover and cook for 30 minutes, stirring occasionally, until the barley is tender and most of the liquid is absorbed.
Stir in the frozen peas and cook for 3 more minutes.
Remove from the heat. Stir in the lemon zest, lemon juice and grated parmesan. Season with salt and pepper.
Serve immediately in shallow bowls.