In a large nonstick skillet, heat the oil over medium-high heat. Add the mushrooms; cook 3 minutes or until mushrooms start to soften, stirring frequently. Add the spinach. Cook and stir 2 to 3 minutes or until spinach is wilted. Set aside.
Butter a 13x9-inch baking dish. Sprinkle half of the bread cubes in the bottom of the dish. Sprinkle with half of the cheese. Spread all of the mushroom/spinach mixture over the cheese. Top with the remaining bread and cheese.
In a large bowl, whisk together the sour cream, eggs, milk, onion powder and salt. Pour over the bread, pressing down to soak the bread. Cover with plastic wrap and refrigerate 2 hours or overnight.
When ready to bake, heat the oven to 350 degrees. Uncover the dish. Bake for 35 to 40 minutes or until knife inserted in center comes out clean.