Preheat oven to 350 degrees F (180 degrees C).
Slice the Onion (1) in half, and then chop into half-circles.
Melt Olive Oil (2 tablespoon) and Unsalted Butter (1 tablespoon) in a pan over medium-low heat.
Place the onion into the pan. Sauté, stirring occasionally, for 15 minutes.
Dice the Bread Boule (1) into medium sized cubes.
Toss the cubes of bread with the Olive Oil (2 tablespoon) and spread onto a baking sheet. Place in the oven for 8 minutes.
Carefully remove the hot baking sheet from the oven and flip the bread cubes. Place back into the oven and bake for another 8 minutes, or until golden brown. Then, remove and set aside.
After the onion have been caramelizing for 5 minutes, add in Balsamic Vinegar (1/4 cup) to the pan and cook, stirring occasionally, for 15 more minutes.
Chop the Cherry Tomato (1 cup) into bite sized pieces and set aside.
Finely chop the Fresh Basil (1/2 cup). Set aside.
In a large bowl, whisk together Egg (8), a pinch of Salt (1 pinch) and Ground Black Pepper (1 pinch) and Milk (1/3 cup).
Add the chopped basil and Mozzarella Cheese (1/2 cup) into the bowl and mix.
When the onion are done, mix them and the tomatoes into the bowl.
Add the toasted bread cubes. Mix until combined.
Spray an oven-proof skillet with cooking spray until well coated.
Pour the contents of the mixing bowl into the pan and cook for 5 minutes over medium-low heat.
Sprinkle the rest of the mozzarella cheese over the top of the frittata and place the pan in the oven. Bake for 20 minutes.
Remove from the oven when the cheese is bubbly and golden. Serve warm.