Preheat and Prep: Position an oven rack in the center and preheat to 375°F (190°C). Lightly coat a 9x13 inch casserole dish with cooking spray.
Sauté Aromatics: Heat the olive oil in a large, deep skillet over medium-high heat. Add the chopped onion and cook for 4-5 minutes, stirring, until it begins to soften.
Cook Sausage: Add the ground turkey sausage, Italian seasoning, smoked paprika, salt, pepper, and optional red pepper flakes. Use a wooden spoon to break up the sausage into small crumbles.
Add Mushrooms: Once the sausage is nearly cooked through (about 5-7 minutes), add the sliced mushrooms. Continue to cook for another 3-4 minutes, until the sausage is fully browned and the mushrooms are tender. Remove from heat.
First Layer: Spread half of the high-fiber bread cubes evenly across the bottom of the prepared casserole dish. Top with half of the sausage/onion mixture, half of the turkey pepperoni, and half of the shredded cheese.
Second Layer: Repeat the process, layering the remaining bread cubes, the rest of the sausage mixture, and the remaining shredded cheese.
Prepare Custard: In a blender or a medium bowl with a whisk, combine the 9 eggs, unsweetened almond milk, and ground flaxseed meal. Blend or whisk vigorously until smooth and slightly frothy.
Pour and Top: Pour the egg mixture as evenly as possible over the entire casserole. Dollop the pizza sauce in spoonfuls across the top. Arrange the remaining turkey pepperoni slices over the surface.
Bake and Rest: Bake uncovered for 40-45 minutes, or until the casserole is golden brown, bubbly, and the center is set (an inserted knife should come out clean). Let the casserole rest for 10 minutes before slicing. Garnish with fresh parsley and serve warm.
