Fry the bacon over medium-high heat until crispy, then transfer to paper towels to drain. Chop or crumble into small pieces and set aside.
Prep the veggies. Dice bell pepper, onion, and tomato and set aside. Slice the scallions thinly, separating the lower (white or white-green) parts of the scallions from the top (green) parts.
Preheat oven to 350 degrees. Crack the eggs into a large nonstick casserole dish.* Add the milk, then whisk until combined.
Add minced garlic, salt, pepper, and 1/4 cup of the shredded cheese to the eggs. Whisk gently until combined.
Sprinkle the diced bell pepper, onion, tomato, sliced scallions (white parts only), and chopped bacon into the casserole dish, making sure to evenly distribute everything. Give the mixture a quick swirl with a whisk or fork to help some of the veggies fall deeper into the casserole dish.
Bake for 25 minutes, or until just solidified. Top with remaining 1/4 cup shredded cheese and sliced scallions (green parts only), then bake for another 15 minutes or until an inserted toothpick comes out clean.
Let cool for at least 5 minutes before slicing into squares and serving.