We wanted to keep this meal super simple so that you could enjoy the fun of a grilled cheese sandwich that is pressed into a waffle maker! Making a grilled cheese sandwich in a waffle maker gives it lots of texture and a crisp golden brown crust. Smarts: Don't have a waffle maker or just don't want to pull it out? Cook the grilled cheese in the typical manner - on a skillet or griddle.
Make vinaigrette - Whisk together mustard, vinegar, and lemon juice. Add olive oil while whisking. Season with some salt and pepper. (Can be done up to 5 days ahead)
Cheese (if not pre-shredded) - Grate cheese. (Can be done up to 5 days ahead)
While waffle iron is heating up, prepare sandwiches by lining up bread on a cutting board. Butter bread on one side. Place cheese between two slices of bread, leaving buttered-side out.
Transfer one sandwich to heated waffle iron, pressing down firmly to leave waffle-shaped imprints in the bread. Toast sandwich until cheese is melted and bread is golden brown. Continue with all remaining sandwiches.
While sandwiches toast, toss together apples, cucumbers, and greens. Add vinaigrette until dressed to your liking.