Preheat your oven to 350 degrees f.
Spray a regular 12 spot muffin tin with cooking spray (or use silicone liners) then divide up your chopped spinach, tomatoes, onions, and cheese equally between all 12 muffin cups.
In a large bowl, whisk together the eggs, milk, sea salt and pepper, and dry mustard.
Pour this egg mixture evenly into the muffin spots over top of all your goodies.
Bake in your preheated oven for 12-15 minutes, or until the eggs are just set and puffed.
Enjoy immediately.
Refrigerate in glass containers for up to 3 days for meal prep.