Preheat your broiler and move the oven rack to about 5 inches from the flame.

In an 11-inch cast iron skillet over medium heat, cook the chopped bacon for about 2 minutes. Add the chopped red onion and continue cooking until the bacon is crispy and the onion is soft, about 5 to 7 minutes.

Meanwhile, whisk together the eggs, salt, pepper, and whole milk in a large mixing bowl until combined and slightly frothy. Set aside.

Remove the bacon and onion to a paper-towel-lined plate. Pour out most of the bacon grease, leaving about 1 tablespoon in the pan.

Add the packed spinach to the skillet and cook over medium heat, stirring for about 1 minute until wilted. Add the minced garlic and cook until just fragrant and lightly golden, about 1 minute. Spread the spinach evenly across the pan.

Scatter the bacon and onion evenly over the spinach. Sprinkle the green onions and shredded gouda on top. Pour the egg mixture evenly over everything. Cook uncovered over medium heat for 6 to 7 minutes until the edges are set.

Using oven mitts, carefully transfer the skillet to the preheated broiler. Broil for 2 to 3 minutes until the top is golden brown and the center is fully set. Watch closely and do not walk away.

Remove from the broiler and let rest for 1 to 2 minutes before slicing and serving. Garnish with reserved green onions if desired.