Brush the sweet potato slices with the olive oil and season lightly with salt and pepper. Heat a ridged griddle pan until hot. Add the sweet potato slices, in batches if necessary, and cook for 3–4 minutes on each side until charred and tender. Remove and set aside. Clean the griddle pan.
Meanwhile, heat a little vegetable oil in a small frying pan, add the sage leaves and fry over a medium-high heat, stirring, for 1–2 minutes until crisp. Remove and drain on kitchen paper.
Cut the ciabatta into quarters, then trim the quarters so that all 4 pieces will fit in the griddle pan. Heat the griddle pan and brush with a little vegetable oil. Add the ciabatta pieces, cut-side down, and cook for 1 minute, or until toasted and charred. Depending on the size of your griddle pan, you may have to toast the bread in 2 batches.
Spread the toasted sides of the ciabatta with the tapenade and sandwich together with layers of fontina, sage leaves and sweet potato slices.
Add the whole sandwiches to the griddle pan and cook for 1–2 minutes on each side until toasted and the cheese in the centre has melted. Serve immediately with a green salad.