Cut the stems off of the mushrooms and scoop out the insides (to make room for the filling), then chop the stems and insides and set aside (you’ll add them to the stuffing later). Transfer the mushrooms to a large mixing bowl and drizzle lightly with avocado oil and sea salt. Use your hands to toss everything together until the mushrooms are coated with oil and salt.

Preheat the oven to 350 degrees Fahrenheit.

Add the avocado oil to a large skillet and heat to medium-high. Add the chopped onion and sprinkle liberally with sea salt. Cover the skillet and cook, stirring occasionally, until the onion begins to turn brown, about 8 to 10 minutes. Stir in the chopped mushroom stems and garlic and continue cooking until the stems soften and turn brown, about another 5 minutes.

Move the veggies off to the side and add the ground beef. Allow it to brown for 2 to 3 minutes per side, then use a spatula to break it into smaller chunks and mix well.

Add the remaining ingredients (oregano, feta, spinach, and sea salt) and stir well. Cover the skillet and cook until the spinach has wilted, about 3 minutes.

Place the mushrooms in a casserole dish and scoop the filling into the mushrooms. If desired, sprinkle with more feta cheese.

Bake on the center rack of the preheated oven for 20 to 25 minutes, or until the mushrooms have softened. Note: moisture will leech out of the mushrooms during the baking process...this is normal.

Transfer the mushrooms to a serving plate or platter and enjoy!