Step 1: Prepare the Vegetables
1. Sauté Vegetables: In a skillet, heat 2 tablespoons of olive oil over medium heat. Add the sliced zucchini, bell pepper, and mushrooms. Season with salt and pepper. Sauté for about 5–7 minutes until the vegetables are tender. Add the spinach and cook for an additional 1–2 minutes until wilted. If using, drizzle with balsamic vinegar and stir to combine.

Step 2: Assemble the Sandwiches
1. Layer Ingredients: On each slice of bread, layer the sautéed vegetables and fresh mozzarella cheese. Top with another slice of bread to form a sandwich.

Step 3: Grill the Sandwiches
1. Heat the Pan: In the same skillet, add a little butter over medium heat.
2. Grill: Place the sandwiches in the skillet and grill for about 3–4 minutes on each side, or until the bread is golden brown and the cheese is melted. Press down gently with a spatula to ensure even grilling.

Step 4: Serve
1. Slice and Garnish: Once grilled, remove the sandwiches from the skillet and let them cool for a minute. Slice in half and garnish with fresh basil leaves if desired.
2. Enjoy: Serve immediately and enjoy your delicious grilled veggie mozzarella sandwiches!