Position your oven rack about 5 inches from the broiler element and preheat the broiler. Have your oven mitts nearby before you start cooking.

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

While the bacon cooks, whisk together the eggs, salt, pepper, and whole milk in a large bowl until fully combined and slightly frothy. Set aside.

Using a slotted spoon, transfer the bacon and onion to a paper-towel-lined plate. Carefully drain most of the bacon grease from the skillet, leaving about 1 tablespoon behind.

Add the packed spinach to the skillet over medium heat. Stir for about 1 minute until it starts to wilt, then add the minced garlic. Cook another 30 seconds until the garlic is just turning golden and the spinach is fully wilted. Spread the spinach evenly across the bottom of the pan.

Scatter the bacon and onion mixture evenly over the spinach. Sprinkle the green onions on top, then distribute the shredded gouda evenly across the skillet.

Pour the egg mixture slowly over the entire skillet. Cook uncovered over medium heat for 6 to 7 minutes without stirring, until the edges look set and firm while the center still has a slight jiggle.

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 just set. Do not step away. Garnish with reserved green onion, slice into wedges, and serve warm.