Put the sundried tomatoes in a small, heatproof bowl and cover with 200ml boiling water; set aside.
Lay the chicken breasts between 2 pieces of clingfilm and use a rolling pin to gently bash the thickest parts to flatten them a little, so the breasts are an even thickness. Season with salt and lightly coat with 4 tsp cornflour. Heat the sundried tomato oil in a large, deep, lidded frying pan over a medium-high heat, then fry the chicken breasts, without turning, for 4-6 mins each side until golden and cooked through. Transfer to a plate and cover to keep warm.
Reduce the heat to medium-low, add the onion and garlic and cook for 10 mins, stirring occasionally, until softened; add a splash of water if it starts to catch. Add the stock and sundried tomatoes; simmer for 1 min. Add the spinach, cover and cook for 2 mins, then remove the lid and stir well.
Mix 1 tsp cornflour with 2 tbsp water in a small bowl. Stir the cornflour mixture, lemon juice and cream into the pan and simmer for 2 mins until thickened. Return the chicken to the pan, cover and cook for 2-3 mins more until the chicken is warmed through, then serve.
