Preheat your oven to 400°F and line a rimmed baking sheet with parchment paper for easy cleanup.
Carefully clean the portobello mushroom caps with a damp paper towel, remove the stems, and use a spoon to gently scrape out the dark gills.
Heat the olive oil in a medium non-stick skillet over medium-high heat.
Add the ground turkey to the skillet and cook until browned, using a wooden spoon to break the meat into very small crumbles.
Stir in the minced garlic and chopped baby spinach, sautéing for 1-2 minutes until the spinach is fully wilted.
Remove the skillet from the heat and allow the mixture to cool for a few minutes so the eggs don't scramble instantly.
Stir in the liquid egg whites, fresh thyme, sea salt, and black pepper until the turkey mixture is well-combined and slightly bound.
Place the mushroom caps on the prepared baking sheet, gill-side up, and divide the turkey stuffing evenly between the two caps.
Crumble the goat cheese over the top of each stuffed mushroom.
Bake for 18-20 minutes until the mushrooms are tender and the goat cheese has softened and turned slightly golden.