Preheat oven to 425°.

Prep by slicing the onions, quartering the cremini mushrooms, and mincing the garlic.

Place a skillet over medium-high heat and add a tablespoon of extra virgin olive oil and two pats of unsalted butter.

When the oil and butter mixture is hot, add the sliced sweet onions, sauté the onions until they begin to brown, turn the heat to low and add another tablespoon of unsalted butter, allow the onions to cook for an additional twenty minutes. Add the quartered cremini mushrooms and minced garlic and sauté for five minutes. Spoon the onions, mushrooms, and garlic into a bowl and set aside. 

Wipe the skillet clean and place it into the oven to allow the skillet to get hot. 
Into a bowl, add the flour, half and half, eggs, whisk until the ingredients are lump-free and smooth. Add the ground garlic pepper and two tablespoons of chopped leeks, ramps, or green onion stems. Mix well. 

Remove the hot skillet from the oven and add two tablespoons of unsalted butter, use a brush and coat the sides of the cast iron skillet, pour the batter mixture into the hot skillet, sprinkle in half of the grated Parmesan.

Place the skillet on the middle rack of the preheated 425° oven and bake for twenty minutes or until the crust is golden brown. 

Remove the skillet from the oven and add the onions, mushroom, and garlic mixture. 

Sprinkle with the remaining grated Parmesan cheese, extra leeks, and fresh chopped cilantro. 

Serve.