Preheat your oven to 400°F (200°C) and line a small baking sheet with parchment paper.
Clean the portobello mushrooms by wiping the caps with a damp paper towel. Remove the stems and use a spoon to gently scrape out the dark gills, then place the caps on the baking sheet, stem-side up.
Finely dice the onion and mince the garlic clove.
Heat the olive oil in a non-stick skillet over medium heat. Add the diced onion and sauté for 3-4 minutes until translucent.
Add the ground turkey to the skillet, breaking it up with a wooden spoon. Cook until browned and no longer pink.
Stir in the minced garlic, sea salt, black pepper, and dried oregano, cooking for 1 minute until fragrant.
Add the baby spinach to the skillet and stir until just wilted, then remove the pan from the heat.
Divide the turkey and spinach mixture evenly between the two mushroom caps, pressing down slightly to pack the filling.
Crumble the goat cheese over the top of each stuffed mushroom.
Bake in the oven for 15-18 minutes until the mushroom caps are tender and the goat cheese is slightly softened and warm.
Let rest for 2 minutes before serving.