Heat the olive oil in a large frying pan. Add mushrooms and cook until the liquid in the pan evaporates. Add spinach or chard and cook until just wilted. Season to taste with sea salt and pepper. Assemble the sandwiches by topping 4 slices of bread with mushrooms and spinach or chard. Add slices of cheddar cheese in a single layer. Top with the remaining bread slices. Brush each side of the sandwich with some soft butter. Grill sandwiches in a heated heavy skillet on medium/high heat. Cook the sandwiches pressing firmly with a spatula for a few minutes on each side until the cheese starts to melt and the sandwich is golden brown. Makes 4 sandwiches. When cooled, wrap in tin foil and place in refrigerator or lunch box. Enjoy this sandwich cold for lunch or heat it up in a microwave for 30 to 40 seconds.