Preheat oven to 350°F. Prepare 8x8-inch baking dish with butter.
Brown sausage in a skillet until cooked through.
Saute green onion and garlic in the same skillet in the rendered sausage fat, or with 1 tablespoon butter until onion is softened and garlic is fragrant 2-3 minutes. Remove from heat.
In a mixing bowl, whisk the eggs, cream, green onion, garlic, salt, pepper, sausage, and green chilies with a fork until combined.
Pour egg mixture into the prepared baking dish, dot with crumbled goat cheese, and bake for 20 minutes. Loosely lay a sheet of foil over the top of the dish to prevent browning, and bake for another 20 minutes. If eggs jiggle in the middle when moved, continue baking until eggs are 'just set' (or at 160°F in the center). Remove from oven and allow to rest for 5 minutes before serving.
Cut into squares and serve with fresh avocado slices and salsa.
Notes
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.