Preheat oven to 400°F (200°C) and line a baking sheet with parchment paper.
Clean the portobello mushrooms with a damp cloth, remove the stems, and gently scrape out the dark gills with a spoon.
Heat the olive oil in a large skillet over medium-high 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 spatula, and cook until browned and no longer pink.
Stir in the minced garlic, dried thyme, sea salt, and black pepper, cooking for 1 minute until fragrant.
Add the chopped spinach and stir until just wilted, then remove the skillet from the heat.
Place the mushroom caps on the prepared baking sheet, gill-side up, and divide the turkey mixture evenly between them.
Sprinkle the grated parmesan cheese over the top of the stuffed mushrooms.
Bake for 15-20 minutes until the mushrooms are tender and the cheese is golden and bubbly.