Heat small skillet over med heat. Spread two slices of bread with the pesto sauce. On top of pesto, layer half of the mozzarella slices, the tomato slices, the spinach and the other half of the mozzarella slices. Top with remaining two slices of bread. Brush top pieces of bread with olive oil. Lay in preheated skillet oil side down. Reduce heat to medium-low. Brush the remaining two slices of bread, now facing up, with oil. Grill each side til golden brown, and cheese is melty.
Tip: Putting a lid on the skillet will allow the cheese to melt faster.