Rub the turkey breast with 2 teaspoons of the oil; sprinkle both sides with salt and pepper. Place on a baking sheet and roast until a thermometer inserted into the thickest part reads 160 degrees F, about 55 minutes. Remove from the oven and let rest 15 minutes; the turkey will continue to cook and the internal temperature should reach 165 degrees F.
Place the bacon lardons on a baking sheet and bake until crisp, about 20 minutes. Transfer the bacon to a paper towel-lined plate; drain and reserve the fat for another use.
Lower the oven temperature to 250 degrees F. Coat the tomatoes with the remaining 2 teaspoons of oil and season with salt and pepper. Place the tomatoes cut-sides up on a parchment paper-lined baking sheet and bake until they start to dehydrate and wrinkle, about 45 minutes.
While the tomatoes are cooking, start the Mornay sauce. Add the butter to a medium saucepan and melt over medium heat. Add the milk to a small saucepan and heat over medim heat. When the butter is melted, add the flour and stir continuously so the roux doesn't scorch, about 2 minutes. When the milk starts to simmer, slowly add it to the roux, stirring continuously so lumps don't form. Increase the heat and bring to a boil, reduce to a simmer and cook until the sauce is thick enough to coat the back of a spoon, 6 to 8 minutes. Add the Cheddar and red peppers and stir until smooth. Remove the sauce from the heat and season with salt and pepper.
You are now ready to assemble the dish. Heat the broiler. Distribute the bread cubes evenly among four 6-inch round cast-iron gratin pans. Cut the turkey into bite-size pieces, about 1/2 inch each, and divide evenly among the gratin pans. Spoon the Mornay sauce over the pans, and top each with 10 tomato halves. Sprinkle each pan with the Parmesan and bacon and place under the broiler on the bottom rack of the oven until golden and bubbly, 5 to 10 minutes. Serve immediately.