Move the onions to one side of the pan and turn the heat up a bit. Add the mushrooms to the empty side of the pan and sauté until they begin to soften, about 5 minutes. (You might need to add a bit of olive oil at this point -- if all of the bacon grease has been absorbed by the onions.) Then mix the mushrooms into the onions, turn the heat to medium and add the sausages, squeezing them out of their casings as you do. Use a wooden spatula to break them into small pieces and incorporate them into the other ingredients. Continue to sauté until the sausage is cooked through, about 8 minutes. Crumble the bacon and add it into this mixture with the spinach. Cook just until the spinach has wilted, about 3 minutes. Season to taste with salt and pepper (here's how
), remove from the heat and set aside.