Heat half the olive oil in a large saucepan over low heat, add chicken thighs, cook for 3 minutes either side or until browned, remove from pan.
Using the same pan; heat remaining oil and add onion and garlic and cook for 5 minutes or until softened. Add chicken to onion mixture, and then add tomato passata, stock, rosemary and bay leaf. Reduce heat to low-medium and simmer for 30 minutes or until tender. Set aside to cool slightly, then coarsely shred chicken, season to taste and keep warm.
Cook spaghettini in a large saucepan of boiling water until al dente (5-7 minutes), drain well, add pasta and baby spinach to sauce and toss to combine. serve hot scattered with grated parmesan