Place the 12 bread slices on the workspace. Onto 6 slices, place the Gruyere cheese and on the other 6 slices, place the smoked salmon. Sprinkle the salmon slices with lemon zest, the Gruyere slices with chives. Place 1 ounce of caviar on the salmon and close the sandwiches and butter the outsides. Preheat a sautepan over medium heat. Add the sandwiches to the pan with the Gruyere side down and saute for 2 minutes. Raise the heat to high, turn them over and saute for 1 minute on the salmon side. Make a vinaigrette by combining the mustard, lemon juice and olive oil. Toss the salad in the dressing to coat and season with salt and pepper. To serve, slice the croque monsieurs on the diagonal, then on the diagonal again and place alongside the salad on a plate.