Preheat your oven to 400°F and line a small baking sheet with parchment paper.
Dice the butternut squash into 1/2-inch cubes and toss with the olive oil, sea salt, and black pepper.
Roast the squash for 20 minutes or until fork-tender and slightly caramelized, then remove from the oven and reduce the temperature to 350°F.
While the squash roasts, dice the sourdough bread into 1-inch cubes and finely crumble the tempeh.
In a non-stick skillet over medium heat, sauté the diced onion, celery, and crumbled tempeh until the vegetables are translucent and the tempeh is lightly browned.
In a medium mixing bowl, whisk together the egg whites, Greek yogurt, vegetable broth, dried sage, and dried thyme until smooth.
In a small casserole dish, combine the sourdough cubes, roasted squash, and the sautéed onion and tempeh mixture.
Pour the egg white custard over the bread mixture, tossing gently to ensure every cube is coated, and let it sit for 5 minutes to absorb the liquid.
Bake for 25-30 minutes until the top is golden-brown and the custard is set.