Cook spelt in boiling unsalted water until tender (18-20 minutes). Drain and set aside.
Heat half the oil in a saucepan over medium-high heat, add onion and sauté until tender (5-7 minutes). Heat remaining oil in a frying pan over medium-high heat, add chicken and cook, turning once, until golden brown. Set aside. Add garlic to pan and stir until fragrant (1 minute). Add to onion with stock and chicken, season to taste, bring to the boil, and simmer until chicken is cooked through (10-12 minutes). Remove chicken, shred meat, then return to stock with spinach and drained spelt. Bring to a simmer, then serve hot, scattered with spinach leaves and parmesan.