Preheat oven to 350 degrees and coat a 9×13 baking dish with nonstick spray or butter. Set aside.
Heat a large skillet under medium heat and add olive oil. Add sliced onions and mushrooms, saute for 5-6 minutes until softened. Add sausage, onion powder, 3/4 teaspoon of salt, 1/4 teaspoon of pepper, and remaining seasonings (sage, marjoram, cloves, red pepper flake). Mix well and cook until meat is browned. Add chopped spinach/arugula, tomatoes, and cook for an additional 3-4 minutes. Pour mixture into prepared baking dish.
Add eggs into a large bowl or blender. Add remaining teaspoon of salt and 1/4 teaspoon of pepper.
Mix egg mixture well until frothy. Pour over vegetable and meat mixture. Bake for 25-30 minutes until edges are browned and center is just set.