Cut OUTER dark green tips off of the leek. Trim off root end, then separate the leaf sheaths; wash thoroughly. Slice leek crosswise into 1/2″ pieces. Set aside.
In a large pot over medium-high heat, cook bacon until crisp. Drain on paper towel & set aside. Remove all but 1 Tbsp of the bacon drippings from the pot. Add the sliced leek & 1/2 tsp sea salt. Stir & cook for about 5 minutes until leek has started to soften. Add 1/4 cup water to deglaze the pot, scraping up any leek bits from the bottom. Reduce heat to medium & cover pot for another 3 minutes, until leek is tender. Turn off heat & keep pot covered.
Preheat oven to 350 F.
In a 9-inch pie pan, scatter cooked potato cubes across the bottom. Top with cooked bacon, gruyere cheese & leek. In a bowl, beat eggs with ricotta cheese, cream, grated Parmesan, sea salt & pepper until smooth. Pour over ingredients in pie pan.
Bake in the middle of the oven for 1 hour, until top is golden brown & center is cooked through. Serve hot or at room temperature. Can be made several days ahead & reheated in the microwave for about 1 minute.