Preheat oven to 400 F. In a large oven proof skillet (like a cast iron skillet), brown sausage and place in a separate container. Drain off extra fat if there is a lot. Place butter in skillet and melt, add in onions, diced potatoes and salt, and cook until onions and potatoes are starting to soften. While those are cooking, place eggs, Italian seasoning and pepper in a separate bowl and whisk until combined. Move to the side. Add in asparagus, garlic, and spinach to the skillet and cook for about 2-3 minutes. Make sure potatoes are cooked, then pour egg mixture over vegetables, cover with shredded cheese, and bake in oven for about 15-25 minutes until eggs are cooked through. Top with fresh chopped herbs right before serving.