Heat the avocado oil in a large skillet over medium-high heat and add the chopped onion. Sauté, stirring occasionally, until the onion begins to soften, about 5 minutes. Add in the fresh minced garlic and sliced sausage links and continue cooking until the sausage begins turning golden brown, about 3 to 5 minutes longer. If adding, add in the baby spinach, cover the skillet, and cook for 2 to 3 minutes, or just until the spinach has wilted.
Preheat the oven to 350 degrees F. Lightly spray a large casserole dish with cooking spray. I use a 13” x 9” casserole dish.
Transfer the chopped bell pepper, olives, and tomatoes to the bottom of a the prepared baking dish large casserole dish. Add in the onion sausage mixture and stir everything together.
Crack eggs into a large bowl and whisk well until the eggs are well-beaten. Stir in half of the grated cheese.
Pour the beaten egg mixture into the casserole dish and use a spoon or fork to stir the egg around until everything is in an even layer and it appears as though the ingredients are well-distributed throughout the dish.
Sprinkle the remaining cheese on top.
Bake on the center rack of the preheated oven for 35 to 45 minutes, or until the edges of the egg bake are golden brown.
Allow the cheesy sausage egg bake to cool for 10 minutes before slicing and serving.
Serve with any of your favorite toppings like extra grated cheese, sour cream (or Greek yogurt), sliced avocado, and salsa.