In large bowl, whisk together eggs, milk, salt and black pepper. Stir in half of the drained tuna.
Heat a 10” oven-proof skillet on medium-high heat. Melt 1 tablespoon of butter and add pancetta. Fry pancetta until crispy. Remove pancetta and set aside.
Add spinach leaves to the rendered pancetta fat. Sauté spinach until wilted, about 1 -2 minutes.
Melt remaining 1 tablespoon butter, then pour in the egg mixture. Stir in egg mixture and then cook untouched for about 20 seconds or until the sides begin to set.
Gently gather in the sides of the cooked egg around edges of the pan to let uncooked egg flow underneath. When the bottom of the eggs is set on the edges, add feta cheese, crispy pancetta, and remaining chunks of tuna on top.
Bake for about 15-20 minutes, or until the frittata is fluffy, light brown on top and the center is just set. Serve warm with toast or grilled bread.