Bring a large saucepan of water to a boil. Add squash and boil until squash is tender. (While the peel adds a nice color and texture, this will work just as well with peeled squash).

Using a sieve or colander, drain the water from the squash. Let squash sit in the sieve for 10 minutes to allow as much water to drip out as possible.

Meanwhile, heat olive oil in a sauté pan over medium heat. Add onion and sauté until translucent, about five to eight minutes. Add mushrooms (optional) and sauté an additional five minutes.

In a large bowl, combine onion/mushroom mixture with the squash. Add mayonnaise, egg, egg whites, salt, pepper, onion powder, and garlic. Mix well. Transfer mixture to a baking dish or individual ramekins. 

Bake for 45 minutes, until squash is golden on top. Mini-soufflés will be ready in 25 minutes.