Preheat oven to 350 degrees F.Place eggs, milk, and salt in a blender and blend 5-6 seconds or until well blended. You can also whisk the eggs by hand but the eggs might not turn out as fluffy.
Spray a non-stick muffin tin with cooking spray, making sure to get each individual cup covered completely. Fill each cup about half full of the egg mixture. You may have a little left over, in that case fill each cup slightly more but don't fill more than 2/3 full.
Evenly distribute the spinach, tomatoes, bacon, cheese, and green onion between the cups. Using this order will distribute the toppings evenly in each cup.
Bake in preheated oven for 10-12 mintues or until eggs are puffy and cooked through. It's a good idea to rotate the muffin pan half-way through to ensure even cooking all around.
Use a knife or thin spatula to remove muffins from cups. Serve hot.