Place salmon on cutting board. Slice crosswise into 1-inch thick strips.
Add oil to a large skillet over medium heat. Add onions, potatoes, carrots, and celery root. Cook 6 minutes, stirring often.
Stir in tapioca starch to coat vegetables. Cook 2 minutes, stirring constantly. Add coconut milk, tomatoes, garlic, stock, bay leaves, red pepper flakes, lemon zest, and thyme. Bring to a simmer. Cook for 8 minutes over a low flame. Season with salt and pepper.
Add salmon, nestling pieces into the vegetables until barely covered with liquid. Return to a very low simmer. Cover. Cook for 8-10 minutes or until salmon is slightly underdone.
Remove from heat. Sprinkle with parsley and feta cheese. Squeeze lemon on top.