Preheat the oven to 375 degrees F. In a large, oven-proof skillet (such as cast iron) over medium heat, cook the bacon until crispy and set aside on paper towels to drain. You can save the fat to cook the rest of the ingredients or pour off the fat and start over with 1 tablespoon of olive oil.
Add the onion to the skillet and saute over medium heat for about 5 minutes, stirring often, until softened. Add the spinach and cook until just wilted, about 2 minutes. Add the garlic and saute until fragrant, about a minute.