For the turkey: Preheat the oven to 350 degrees F. Drizzle with the olive oil and sprinkle generously with salt and pepper over the whole turkey breast. Place on a roasting pan and cook until it begins to brown, about 50 minutes. Raise the temperature to 400 degrees F and roast until the skin is golden brown and crispy and an instant-read thermometer inserted into the middle of the breast reads 165 degrees F, about 15 minutes more. Remove the turkey from the oven and let cool. Cut into 1/4-inch-thick slices.
For the mornay sauce: Cook the butter and flour over medium heat in a pot, stirring continuously for 5 minutes; the roux will be pale gold in color. Raise the heat to medium-high and add the milk. Whisk until the sauce thickens, about 5 minutes. Season with salt and pepper. Remove the pot from the heat and stir in the Cheddar, pecorino and nutmeg.
For the sandwich: Preheat the broiler. Assemble the hot brown by laying the toast down in a large pan. Divide the sliced turkey over the top. Layer with the tomatoes, mornay sauce and pecorino. Place the pan under the broiler until the sauce is golden brown, about 5 minutes. Transfer to a plate and sprinkle with the paprika and parsley and top with the bacon.