Preheat oven to 400°F. Prick potatoes a few times with a fork and place on a sheet pan. Bake until cooked through and a fork pressed into the center goes in easily, about 45-60 minutes, depending on size. Let cool slightly. (See Notes)

(Optional) Heat a skillet over medium high heat and cook bacon until crispy, about 4-5 minutes, stirring often. Transfer to a paper towel to drain.

Melt butter in skillet over medium heat (it can be the same skillet you cooked bacon in, just drain off the grease). Add garlic and cook for 2 minutes. Add spinach and a pinch of salt and cook, stirring frequently, until spinach is wilted, about 2 minutes.

Cut potatoes in half and use a spoon to scoop the baked potato out of the the skins. Place potato flesh in a large bowl and mash, set skins back on the sheet pan.

Stir in milk and salt and pepper and stir until mostly smooth. Add in spinach mixture, bacon (if using) and half of the cheese.

Spoon potato mixture into the potato skins. Sprinkle with remaining cheese.

Reduce oven temperature to 375°F. Bake until cheese is melted and bubbly, about 20-25 minutes.