Heat half the oil in a heavy-based saucepan on medium heat. Add chicken and cook for 2-3 minutes, until browned. Transfer to plate. Cover and keep warm.
Heat remaining oil in the same saucepan on medium. Sauté mushrooms, leeks, onion, and garlic for 1-2 minutes, until starting to soften. Season to taste. Stir rice through. Cook for 1 minute.
Stir in 1 cup of hot stock and cook, stirring, until stock is absorbed. Continue adding stock 1 cup at a time until each addition is absorbed, stirring, and simmering for about 20-25 minutes, until risotto is tender and creamy.
Stir in chicken, cherry tomatoes, spinach, parmesan and lemon. Add chopped herbs. Cook for a further 2-3 minutes and then serve.