Step 1: Grease a 9x13-inch baking dish. Preheat oven to 350°F (175°C). If using fresh vegetables or raw meat, sauté them in 1 tablespoon of olive oil or butter until tender and cooked through. Drain any excess liquid or fat.
Step 2: In a large bowl, whisk together the eggs, milk (or half-and-half), salt, and pepper until well combined and slightly frothy.
Step 3: Sprinkle half of the cooked meat (if using) and half of the cooked vegetables evenly over the bottom of the prepared baking dish. Top with 1 cup of the shredded cheese.
Step 4: Carefully pour the egg mixture over the ingredients in the baking dish, ensuring everything is evenly distributed.
Step 5: Sprinkle the remaining 1/2 cup of shredded cheese over the top of the egg mixture. At this point, the egg casserole recipe can be covered and refrigerated overnight.
Step 6: Bake the egg casserole for 40-50 minutes, or until the center is set and a knife inserted comes out clean. The top should be golden brown. If baking from refrigerated, you might need to add 10-15 minutes to the baking time.
Step 7: Remove from the oven and let the egg casserole rest for 5-10 minutes before slicing and serving. This resting period helps the casserole set, making it easier to cut clean servings.