Preheat the oven to 350 degrees Fahrenheit.
Heat the avocado oil in a large skillet over medium-high heat. Add the onion and sauté, stirring frequently, until the onion begins to turn golden-brown, about 5 to 8 minutes.
Transfer the breakfast sausage to the center of the skillet, scooting the onion off to the side. Mash it down to allow much surface area to brown and allow it to brown for 2 to 3 minutes. Flip the sausage and allow it to brown another 2 minutes before using a spatula to chop it up into smaller bite-sized pieces.
Add in the garlic and spinach and cover the skillet. Cook for 2 minutes, or until the spinach has wilted.
Transfer the sausage spinach mixture to a large (13” x 9”) casserole dish.
Add the sun-dried tomatoes, kalamata olives, artichoke hearts and feta cheese and give everything a stir.
Pour the beaten eggs over everything evenly. No worries if it isn’t perfect - you can move the ingredients around to distribute them evenly using a spoon.
Bake on the center rack of the preheated oven for 40 to 45 minutes, or until the casserole has set up and tests clean.
Allow the casserole to cool for 15 minutes before slicing and serving.