Heat a large non-stick skillet over medium heat. Add in the oil to heat then lay in the sliced cheese. Allow the cheese to melt and get golden brown on the bottom – this should take about 2 minutes.
Meanwhile, toast the bread. Cut a small piece off the end of the garlic clove and rub the cut end on the hot toast. Spread the toast with mustard and lay on the grilled cheese, golden side down.
Add on the tomato slices and season with salt and pepper. Scatter with chives, if using, and serve immediately.