Toss 2 tablespoons of the oil with the sweet potatoes. Season well and cook in a preheated hot griddle pan for 3 minutes on each side until tender and lightly charred.
Meanwhile, mix together the remaining oil with the white wine vinegar and season to taste. Toss with the spinach and red onion and arrange on serving plates. Arrange with the sweet potatoes, feta, chilli and olives and serve immediately.