In an oven-safe skillet, over medium heat, add the mushrooms to the skillet with about 1/4 cup of water.

When the water is boiling, add the diced onion. Season with a pinch of Kosher salt. Cook, stirring occasionally until the water is evaporated.

Add a drizzle of olive oil (about 1 tbsp.). Let the mushrooms develop a little browning, just about 3-4 minutes. Then add the spinach, one handful at a time, stirring to let each batch of spinach wilt between additions.

Once all of the spinach has been added and is wilted, add the chopped garlic. Season with another pinch of Kosher salt. Stir and let cook about 30 seconds to 1 minute, just until the garlic becomes fragrant. Turn off the heat.

While the mushroom and spinach mixture is cooling slightly, beat the eggs with the two tablespoons of cream.

Add the spinach and mushroom mix to the eggs.

Then add crumbled feta and scallions to the egg mixture and stir to combine.

Wipe out the skillet if needed, then set the burner to medium/medium-high heat.

Add 1-2 tablespoons of olive oil to the skillet. When the oil becomes shimmery, pour the egg mixture into the skillet.

As soon as you see the edges beginning to set, use a silicone spatula to lift up the set edge of the egg mixture and tilt the pan to let the runny egg fill in the gap. Do this all around the edge of the frittata as the edges begin to set.

After letting the runny eggs fill in the gaps around the edges, and the edges have begun to set again, but the top is still uncooked, reduce the heat to low and place a lid on the skillet.

Set an oven rack to about 6 inches below the heating element and turn the broiler on. Let the frittata continue to cook, covered, over low heat for about 5 minutes. Check it at this point. We want the bottom layers to be set and cooked, but for there to be a little bit of uncooked egg on top. If it's still very undercooked throughout, let it go another 5 minutes.

Run a silicone spatula around the edge of the frittata, sliding it under the frittata to be sure the bottom isn't stuck to the pan. Then turn off the burner.

Place the pan, uncovered, in the oven and broil for about 2 minutes. Watch it closely. As soon as the top begins to bubble, let it puff up and brown in some spots, then remove from broiler. Let the pan cool for about 5 minutes. Run the silicone spatula around the edges and under the frittata to ensure it's not stuck. Then cut it into wedges and serve. Top with fresh cracked black pepper and flaky finishing salt, if desired.