Preheat oven to 350°F. Prepare 8x8-inch baking dish with butter.
Saute green onion and garlic with 1 tablespoon butter in small skillet until onion is softened.
Mix eggs, cream, green onion, garlic, salt, pepper, sausage, and green chilies in a mixing bowl until well combined.
Pour egg mixture into prepared baking dish. Dot with crumbled goat cheese and bake for 30-40 minutes. If eggs jiggle in the middle when moved, continue baking until eggs are 'just set', but not browned. Remove from oven and allow to rest for 5 minutes before serving.
Garnish each serving with fresh avocado slices, tomato, and feta cheese, as desired. Salt and pepper, to taste.
Any combination of add-ins can be substituted for the sausage, green chilies, and goat cheese.
If using firmer cheeses like cheddar, mix into the eggs rather than sprinkling on top before baking.
Before adding chopped vegetables, sauté in a skillet with butter for 3-5 minutes to soften and allow moisture to evaporate before adding to the eggs.