Preheat the oven to 350°F (175°C). Lightly grease a 13” x 9” casserole dish.
Place a large skillet over medium-high heat and add the 15 mL of avocado oil. Add the chopped onion and diced red bell pepper, sautéing until they begin to soften, about 5-6 minutes.
Add the ground chicken sausage and minced garlic to the skillet. Cook, using a spatula to break the meat into small pieces, until the sausage is fully browned, about 6-7 minutes.
Remove the skillet from the heat. Add the entire package of baby spinach and stir into the hot sausage mixture. Continue stirring until the spinach has wilted from the residual heat.
Spread the sausage and vegetable mixture evenly into the bottom of the prepared casserole dish. Scatter the rinsed artichoke hearts, rinsed olives, and sun-dried tomatoes over the top.
In a large bowl, whisk together the 11 eggs, ground flaxseed, dried oregano, and black pepper until well-combined. Pour this egg mixture as evenly as possible over the ingredients in the dish. Gently sprinkle the crumbled feta cheese over the top.
Bake for 40 to 45 minutes, or until the center of the casserole is firm to the touch and a knife inserted in the middle comes out clean.
Allow the casserole to rest and cool for at least 10 minutes before slicing into 8 squares and serving.