Heat olive oil in large skillet or French oven over medium head. Add chopped onion and garlic. Sauté for 3-4 minutes until soft and fragrant.
Add pearl barley to pan and stir for 1-2 minutes to lightly toast.
Stir in oregano and bay leaves.
Pour in stock. Bring to a gentle boil, then reduce heat to low and cover with a lid. Simmer for 20 minutes, stirring occasionally, until the barley is tender and chewy.
Add mussels directly to the pot. Cover with a lid and cook for 5-7 minutes, until the mussels open. Discard any that remain closed. Remove mussels from the pot, set aside and cool slightly, then take the meat out of the shells.
Stir in chickpeas and cherry tomatoes. Cook for 5 minutes to warm through.
Fold in broccolini and cover for 3-5 minutes until bright green and tender-crisp.
Stir in frozen peas and lemon juice. Cook for 2 more minutes.
Gently fold in cooked mussels and heat just until warmed through. Remove bay leaves.
Divide risotto among bowels and garnish with a squeeze of lemon. Serve warm.