Preheat the grill to medium.
Place the sweet potato into a pan of boiling water and cook for 5-6 minutes, or until tender, but not completely cooked, then drain and set aside.
Heat the oil in an ovenproof frying pan over a medium heat. Add the onion and fry for 3-4 minutes, to soften.
Add the tomato and spinach and cook for 2-3 minutes, until the spinach has wilted.
Add the sweet potato and stir well.
Add the beaten eggs and season well with salt and freshly ground black pepper. Cook, stirring occasionally, until the egg starts to set.
Transfer to the grill and cook for 3-4 minutes, or until the top of the tortilla has turned golden-brown and the tortilla is cooked through.
To serve, carefully turn the tortilla out of the pan onto a plate and cut into wedges.