First, preheat your oven to 375°F so it is ready when you finish assembling the dish.
In a large skillet, heat the olive oil over medium heat. Next, add the diced onion and bell pepper and sauté for 3 to 4 minutes until they start to soften. Next, stir in the finely minced garlic and let it cook for about one minute so that its enticing aroma fills the pan.
Now, add the lean ground beef to the skillet and cook until it browns thoroughly. As you stir, season with salt, black pepper, paprika, and dried oregano. Then, stir in the drained diced tomatoes and the optional spinach; let the mixture simmer for 2 minutes so the flavors blend well.
In a separate large bowl, whisk together the eggs and milk until well combined. After that, gently fold in the thawed hash browns and the cooked beef mixture. Then, mix in half of the shredded cheese, ensuring even distribution.
Transfer the combined mixture into a greased 9x13 inch baking dish. Next, sprinkle the remaining cheese evenly over the top.
Place the dish in the oven and bake for 35 to 40 minutes or until the casserole is hot throughout and the cheese has melted with a light golden color on top.
Finally, remove the casserole from the oven and allow it to rest for 5 minutes before serving. This step lets the flavors settle and the texture firm up slightly.