Beat eggs and sour cream well. Pour into a greased 1 1/2-quart pan and bake at 400°F just until set, about 7 to 10 minutes. Don't overcook, allow to cool slightly. Meanwhile, sauté sausage, chopped onion and mushrooms. Drain on paper towels. Spread sausage over cooked egg. Spread salsa over all. Layer cheddar and Jack cheese. This casserole may be refrigerated at this point. To serve, bake uncovered at 300°F for 30 minutes. Remove from the oven and allow to sit for 10 minutes before cutting into squares to serve.
GARNISH: Arrange rings of chopped green onion, chopped tomato, and black olives. Top with sour cream. Arrange taco chips standing around the outside edge of casserole, if desired.