Toss the bacon bits into a cold 12-inch skillet and crank the heat to medium. Cook, stirring occasionally, until the bacon bits are crispy, around 10 to 12 minutes.
Once the bacon bits are crispy, use a slotted spoon to transfer the bacon bits to a paper towel-lined platter.
Toss the shallots in the pan with the reserved bacon grease. Sauté the sliced shallots until they’re translucent and softened, about 1 to 2 minutes.
Add the mushrooms into the pan. Season with ¼ teaspoon kosher salt and ⅛ teaspoon black pepper and cook until they've developed some brown bits and the liquid has evaporated, about 5 to 7 minutes
Stir in handfuls of the spinach in batches, adding in more spinach as the previous batch wilts.
Once all the spinach has been added, season the dish with ½ teaspoon salt, and ⅛ teaspoon pepper. Continue stir-frying the spinach until it has just wilted. Turn off the heat and mix in the vinegar.
Transfer the spinach to a plate and sprinkle the bacon bits on top. Enjoy!