Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
Clean the portobello mushrooms by wiping them with a damp cloth, then remove the stems and gently scrape out the dark gills with a spoon.
Mince the garlic and finely chop the fresh parsley and basil.
In a skillet over medium heat, add the olive oil and ground turkey, breaking it up with a spatula until browned.
Stir in the minced garlic, sea salt, black pepper, and red pepper flakes, cooking for another 1 minute until fragrant.
Add the fresh spinach to the skillet and sauté until just wilted, then stir in the fresh parsley and basil.
Place the mushroom caps on the prepared baking sheet, stem-side up, and divide the turkey mixture evenly between them.
Bake for 15-18 minutes until the mushrooms are tender and juicy.
Remove from the oven, sprinkle with crumbled feta cheese, and drizzle with fresh lemon juice before serving.