01 - Preheat oven to 350°F. Grease a 9x13-inch baking dish lightly with cooking spray or butter.
02 - Heat olive oil in a skillet over medium heat. Add diced potatoes and cook for 5–7 minutes until beginning to soften. Add onion and bell pepper, continue cooking for 3 minutes. Stir in spinach and cook until wilted, about 1–2 minutes. Remove from heat and set aside.
03 - In the same skillet, cook diced bacon over medium-high heat until crispy and browned, approximately 6–8 minutes. Transfer to paper towels to drain excess grease.
04 - Crack eggs into a large mixing bowl. Add whole milk, salt, black pepper, and smoked paprika. Whisk vigorously until mixture is uniform and slightly frothy, about 1 minute.
05 - Spread sautéed vegetables evenly across the prepared baking dish. Add cooked bacon and bread cubes if using. Sprinkle half the shredded cheese over the mixture.
06 - Pour the egg mixture evenly over the layered ingredients, ensuring distribution reaches all corners. Top with remaining shredded cheese.
07 - Place dish in preheated oven and bake for 35–40 minutes. The casserole is done when eggs are completely set, the center no longer jiggles, and the top is golden brown.
08 - Remove from oven and let rest for 5 minutes to allow the casserole to set for easier serving. Garnish with fresh chives or parsley. Cut into squares and serve warm.