Preheat oven to 375°F and spray a 12-cup standard muffin pan with cooking spray.
Melt butter in a medium skillet over medium-low heat.
Add bell pepper, spinach, tomato and shallot; cook and stir until softened; about 5 minutes.
Arrange bread on work surface and use a rolling pin to flatten each slice.
Cut bread diagonally in half and place into each muffin pan well, forming a cup.
In a medium bowl, whisk eggs, mustard, Italian seasoning and black pepper.
Stir in vegetable mixture and bacon.
Slowly ladle egg mixture to the top of each well, being careful not to overfill.
Sprinkle evenly with cheese and chives.
Bake 15 minutes or until eggs are set.
Let stand 5 minutes before gently using a knife to loosen toast bites from muffin pan.
Garnish with additional chives, if desired.