Heat 2 tablespoons of the reserved bacon drippings in a 10-inch skillet, and place over medium heat. Add the sliced mushrooms, season with fresh black pepper, and cook until all of the moisture has been released and has evaporated. Remove the mushrooms from the pan and return to the heat.
Add the spinach to the pan and toss, cooking just until it is slightly wilted, but still bright green. Remove from the pan and turn off the heat.
In a large bowl, whisk together the eggs until completely blended, seasoning them with salt and fresh black pepper. Add all of the remaining ingredients into the bowl with the eggs and fold together with a spatula until everything is evenly distributed and all of the bread has been moistened.
Turn the mixture into a 1½-quart baking dish, pressing down if necessary (the mixture will sit about 1-inch about the top of the dish). Sprinkle the top with the cheddar cheese and bake for 15 to 20 minutes, until the strata is set and the top is golden brown. Sprinkle chopped chives on top and serve. Leftovers can be stored in the refrigerator for up to 3 days.