To begin the creamed leeks, heat the olive oil and butter in a medium saute pan over medium heat and saute the onions until translucent. Deglaze the pan with wine and add the leeks, cooking gently until the liquid is almost all gone. Make a slurry by gradually adding the cornstarch to the cream, and then whisk in the Parmesan. When the wine has mostly evaporated, stir the slurry mixture into the leeks, season with salt and pepper, and reduce heat to a simmer to allow reducing and thickening. Set aside in a warm place.
Slice each fish fillet in half lengthwise. In a plastic bag, shake together cornmeal, flour, garlic pepper seasoning, and salt. Moisten fish by dipping in milk and shake in bag to coat with cornmeal mixture. Heat olive oil and butter over medium heat in a large skillet, and cook each side of coated fish until golden brown. Do not overcook the fish - it is done when it springs back upon gentle prodding.
Spoon some leek ragout in the middle of serving plate, top with tilapia, and garnish with parsley and lemon slices.