Cut top third off garlic heads and discard. Wrap each head in foil. Bake until very soft, about 50 minutes. Let cool. Remove foil. Squeeze out pulp into a small bowl.
Combine garlic pulp and tomato paste in a large saucepan. Stir in stock. Bring to the boil. Add the turkey, rice, spinach, pepper and chilli flakes. Simmer, uncovered, 8 minutes. Just before serving, stir in lemon juice.