Preheat oven to 180°C and lightly grease the loaf pan.
Mix flaxseed with water and rest five minutes until thickened.
Sauté mushrooms and spinach with olive oil and garlic powder for 3 minutes, then cool slightly.
Combine protein base, oats, onion, garlic, milk, flax mixture, tomato paste, oil, soy sauce, and spices evenly.
Press half the mixture into loaf pan, add stuffing evenly, then cover with remaining mixture.
Bake for 45 minutes until firm, rest five minutes, then slice carefully.