Meanwhile, heat a large, heavy skillet over medium. Once hot, add butter and olive oil and once they’re fully melted and a bit sizzly, add the leek slices, still wet. Sprinkle with 1/2 teaspoon coarse salt and a few grinds of black pepper. Reduce heat to low, cover with a lid and cook leeks for 25 minutes, stirring them occasionally. Adjust seasoning to taste.