Preheat oven to 180C/350F.
Whisk eggs, milk, salt and pepper.
Heat oil in a well seasoned skillet or non stick pan (about 26cm / 11") over medium high heat.
Add bacon and cook until almost golden.
Add garlic and cook for 30 seconds.
Add spinach and toss until just wilted - bacon should now be golden.
Turn heat down to medium. Remove about ⅓ of the bacon spinach into a bowl (for topping).
Add butter into the pan and swirl/spread across base of the pan.
Pour in about ⅔ of the egg mixture. Scatter over half the cheese, then pour over remaining egg mixture.
Scatter over reserved bacon and spinach, then cheese.
Cook for about 3 minutes until the sides are just set but not golden - check with knife.
Transfer to oven and cook for 8 minutes until the centre is just set and the cheese is melted on top.
Rest for a few minutes then slice and serve.
