1. Preheat oven to 350 degrees F. Line 18 standard-sized muffin cups with paper liners and coat with non-stick cooking spray. Using your hands squeeze a 10-oz. pkg. frozen, thawed, spinach until all excess moisture is removed. Coarsely chop spinach.
2. In a large bowl, whisk together 12 eggs, 1 cup milk, 1 cup chopped ham, 1 cup shredded Cheddar cheese, 2 Tbs. minced fresh chives, and 1 tsp. kosher salt. Cut 4 slices whole-wheat sandwich bread into 1/2-inch squares (3 cups). Stir chopped spinach and bread cubes into egg mixture.
3. Spoon about 1/3 cup egg mixture into each cup. Bake until set (160 degrees F), about 18 minutes. Let cool 10 minutes before serving. Pack 2-3 stratas per box.