For the frittata: Preheat the oven to 375°F (190°C). In a medium bowl, whisk together the eggs, milk, shredded cheese, salt, and pepper until well combined.

1. Cook the vegetables: Heat olive oil or butter in a skillet over medium heat. Add the vegetables and sauté until softened (5-7 minutes). If using leafy greens, ensure they are cooked until wilted and excess moisture evaporates.
2. Cook the frittata: Pour the egg mixture over the cooked vegetables. Let it cook on the stovetop for 3-4 minutes until the edges begin to set. Then transfer to the oven to cook for 10-12 minutes until the eggs are fully set and golden. Alternatively, cook on low heat with a lid until fully cooked.
3. Cool and slice: Let the frittata cool slightly, then slice into quarters or halves for your sandwich.
4. For the sandwich: Toast the bread slices with butter or olive oil in a toaster, grill pan, or oven until golden and crispy.
5. Assemble the sandwich: Spread butter or mayonnaise (optional) on one slice of toasted bread. Place a slice of frittata on the bread and top with fresh greens, sliced tomato, avocado, and condiments like mustard or hot sauce.
6. Top and serve: Add the second slice of bread on top, press gently, slice the sandwich in half, and serve immediately.