01 - Preheat oven to 375°F. Line a baking sheet with parchment paper.
02 - Cut croissants in half horizontally. Arrange cut side up on prepared baking sheet.
03 - Whisk together eggs, milk, salt, and pepper in a medium bowl until well combined.
04 - Melt butter in a nonstick skillet over medium-low heat. Pour in egg mixture and gently scramble with a spatula until just set but still slightly creamy. Remove from heat.
05 - Place ham and cheese on bottom half of each croissant. Top with scrambled eggs. Add tomato, avocado, and chives if desired. Replace tops and press gently.
06 - Warm assembled sandwiches in oven for 5–7 minutes until cheese is melted and croissants are crisp. Serve immediately.