Preheat the oven to 400 degrees F. Remove the skins from the onions - leave the root intact but trim it off a little to get it to stand without going through the bottom.
Rub them lightly all over with olive oil, salt and pepper. Wrap them in foil.
Place them, standing up, on a baking sheet and bake about 40 minutes. Remove and let cool until you can handle them.
Turn the oven down to 350 degrees F. Lay the bacon out on a small foil-lined baking sheet or pan and bake until crispy about 15 minutes. Drain on paper towels and chop.
Turn the oven back up to 400 degrees F.
In a medium bowl toss the bacon, cheese and spinach together with a couple dashes salt and pepper.
Use a paring knife to hollow out the center of the onions. Start in the center with a small cut and work your way outward. You should be able to easily pull the outer pieces right out.
Chop up a couple tablespoons of the onions and mix it into the filling.
Divide the mixture evenly among the onions packing it in so it all fits.
Bake, uncovered, about 20 - 25 minutes until the cheese is bubbly.