In a large heavy bottom pot over medium heat add your butter, olive oil, shallots and pancetta and cook until the pancetta is very crispy. This is important. You want it to the point it is just starting to brown like crispy bacon (5-10 minutes).
Now add your baby spinach to the pot and mix well making sure to coat your spinach in the pancetta and olive oil mixture. Cook until your spinach just begins to wilt.
Remove from heat.
Now add your salt and toss your spinach around in the pan again just to make sure you get all of the bits of pancetta and shallot from the bottom of the pan incorporated into your spinach.
Now squeeze ½ of your lemon over the spinach and toss again.
Pour into a serving bowl making sure to get all of the yummy juices and bits of pancetta and shallots at the bottom.