Cook elbow macaroni according to package instructions. Drain, rinse and immediately add the pasta back into the same pot. Over low heat, immediately add the gruyere, butter, salt, green onions or chives and the heavy cream. Mix thoroughly until the macaroni has come together and the cheese has melted. Turn off the heat and set aside.
Butter one slice of bread on both sides and set aside. Using a cast iron skillet or non-stick skillet, heat one tablespoon of butter in your skillet over medium heat. Let the butter melt completely and then lay your slice of buttered bread into the skillet and top with a ¼ cup of the grated Emmentaler cheese. Cover the skillet with a lid and let the cheese melt until it's almost entirely melted, for about 2-3 minutes.
Next, lift the lid and top the melted cheese with the macaroni and cheese and cover again with the lid for a minute or two. Butter one side of the second slice of bread and spread dijon mustard on the other side. Place the bread, dijon mustard side down onto the macaroni and cheese and flip your grilled cheese to begin toasting the other side. You do not need to cover the pan again since your cheese is already melting nicely. You can flip your grilled cheese sandwich a couple more times if desired until you have nice, golden brown toasty bread on both sides.
Repeat everything for your second grilled cheese sandwich.