Preheat oven to 350 degrees F (175 degrees C). Grease a 1-quart baking dish.

Bring a large saucepan of water to a boil. Cook peppers in boiling water until slightly tender, about 3 minutes; drain and rinse with cold water until cooled.

Heat oil in a skillet over medium heat. Cook ground turkey, onions, mushrooms, and garlic in the hot oil until turkey is completely browned; drain grease from the turkey mixture. Transfer drained turkey mixture to a bowl and return skillet to medium heat.

Melt butter in the skillet; add flour, salt, and black pepper. Stir until smooth. Stream milk and half-and-half into the flour mixture while stirring; bring to a boil. Cook and stir the milk mixture until thickened, 1 to 2 minutes. Return turkey mixture to the skillet; add tomatoes and 2 tablespoons Cheddar cheese. Cook and stir the turkey mixture until hot and the cheese is melted, about 3 minutes; spoon into the bell pepper halves.

Arrange stuffed pepper halves into the prepared baking dish; top with remaining Cheddar cheese, Parmesan cheese, and panko. Cover dish with aluminum foil.

Bake in preheated oven until the peppers are tender and the filling is hot, 25 to 30 minutes.