Preheat oven to 350.
First, cook the bacon: chop and add to cold skillet. Bring it up to medium-high heat and cook until just brown and crispy. Remove from pan with slotted spoon and set aside. Reserve bacon grease in pan.
Add onions to same skillet and saute until translucent, about 5 minutes. Stir occasionally.
Grate cheese while you wait.
Lower the heat and add the spinach. Cook until it wilts. Add bacon back to the skillet and stir everything so it's evenly spread in the pan.
Turn off the heat.
Crack eggs into a bowl and whisk together til a bit frothy.
Add milk and salt. Whisk again.
Pour eggs over spinach, onion, and bacon, and sprinkle in grated cheese. Stir everything gently until it's all evenly distributed. Crack some fresh black pepper evenly over the top.
Turn heat back on to medium-high and let the frittata set over the heat. Do NOT stir it anymore.
After about 5 minutes or until it looks a bit done around the edges, remove from stove and put it in the oven.
Let it cook undisturbed for 15 - 18 minutes. Keep an eye on it so it doesn't dry out. You can insert a knife in the middle to check for doneness. The more eggs you use, the longer it will take.
Remove from oven, and garnish with chopped scallions.
Cut into 8 pie slices and serve!