1. Coat pan or grill with oil or butter. Cook vegetables (zucchini, mushrooms, onion, and tomato) to desired tenderness.
2. "Honey" Mustard Sauce: In a small bowl, mix lemon juice, mustard, olive oil, agave nectar, apple cider vinegar, garlic powder, salt, and pepper. (This can be as easy or as complicated as you want. If you're in a hurry, try mixing together a larger squirt of mustard, a smaller squirt of agave nectar, a drizzle of olive oil, and then add apple cider vinegar until it's sauce-y enough.)
3. Layer one piece of bread with vegetables. Drizzle with dressing and cover with other slice of bread. Simple but delicious!
I used a george foreman type grill to do this and it worked great!
Source of recipe: Just messed around and came up with it