Melt the butter and olive oil over a medium-high flame in a large sauté pan. Once the oil and butter are hot, place the leeks cut side down into the pan. Let the leeks brown in the pan for 4 to 5 minutes. Carefully flip the leeks over and turn the heat on low. Cover and let the leeks braise for about 25 to 30 minutes or until the leeks are soft all the way through. Take the leeks off the heat and squirt the lemon juice over the braised leeks and add salt and pepper to taste. Sprinkle with the panko topping.
For the panko topping: Combine panko with parsley, Parmesan, and salt and pepper in a small dish. In a dry skillet over medium heat, toast this mixture until golden brown. Serve over the leeks. Make sure that you taste the panko mixture for correct seasonings to ensure that the dish is seasoned all the way through.