Preheat the oven to 350°F and spray a 2 quart shallow baking dish with cooking spray. Set aside.
In a large skillet cook the bacon over medium-high heat until crisp. Remove to paper towels to drain. Reserve 3 Tbsp drippings.
Add the sliced mushrooms and onion to drippings. Cook for 3-5 minutes over medium-high until softened and beginning to brown.
Ring the spinach in a kitchen towel to remove all excess moisture, then add to the pan. Add salt, black pepper, red pepper flakes, half & half, heavy cream and 1/4 cup grated Parmesan cheese. Mix well.
Add 1 1/2 cup shredded cheese. Crumble the bacon and add back to the spinach reserving 2 Tbsp for the top. Mix well then pour into the baking dish.
Sprinkle with the remaining 1/2 cup shredded cheese.
Toss the panko breadcrumbs, 1/4 cup grated Parmesan cheese with 2 Tbsp melted butter and sprinkle on top of the cheese.
Bake for 30 minutes or until golden and bubbly.
Garnish with reserved bacon crumbles before serving.