Preheat the oven to 350°F and position a rack in the center. Lightly grease a standard 12-cup muffin tin with nonstick cooking spray. Evenly distribute the chopped spinach, red bell pepper, green bell pepper, and tomatoes into the muffin cups. They should be approximately two-thirds full, allowing space for the eggs to expand while baking.
In a large mixing bowl or a measuring cup with a spout for easy pouring, vigorously whisk together the eggs, egg whites, kosher salt, basil, oregano, and black pepper until well blended. Carefully pour the egg mixture into each muffin cup, filling them about three-quarters of the way to ensure they don’t overflow during baking. Sprinkle crumbled feta evenly over the tops.
Place the muffin tin in the oven and bake for 24 to 28 minutes, or until the egg muffins are fully set and slightly golden around the edges. Remove from the oven and let them rest for a few minutes before running a butter knife around the edges to gently loosen them. Serve warm straight from the pan, or transfer to a wire rack to cool before storing in the refrigerator or freezer for later enjoyment.