Spread some plant-based margarine one side, of all four slices of bread, and place 2 slices, margarine side down in the frying pan.
Working quickly, on each slice of bread: spread 1 Tbsp (15 ml) pesto, followed by a slice of cheese, ½ the sliced avocado, ½ the chopped spinach, salt/pepper, and more cheese. Finally place the remaining slices of bread, butter side up, on top.
Cover the pan (so cheese will melt) and cook about 3-5 minutes – you want the bread to brown, but not burn. Flip the sandwiches and repeat.