In small saucepan, bring milk to a simmer. In a medium saucepan, melt the butter and then add the flour and whisk for a minute. Gradually add the hot milk, whisking constantly, and bring to a boil. Cook and whisk until thickened, about 5 minutes or so. Remove from heat and add the cheddar and parmesan cheeses, whisk until melted and smooth. Add the nutmeg and hot sauce and season with salt and pepper. Cover and set aside.
Preheat oven to 425º. Arrange bacon on a rack over a sheet pan and bake until crisp, about 30 minutes (or fry on the stove, both ways make a mess!).
Preheat the broiler and arrange tomato slices on a sheet pan, drizzle with canola oil, and some salt and pepper. Broil 6 inches from heat, 1 to 2 minutes per side, until lightly charred.
Arrange 6 slices of bread on a baking sheet. Spread ½ tablespoon of butter on each slice. Broil until lightly toasted. Watch very carefully so it doesn't burn. Takes about a minute. Flip bread and lightly toast other side.
Top each toast with 2 slices of turkey and a tomato slice. Spoon the cheese sauce on top and sprinkle remaining cheddar and parmesan evenly over the top. Broil until cheese is melted, golden brown and bubbly, about 2 to 4 minutes.
Transfer sandwiches to platter, or keep on sheet pan, and top each hot brown with 2 slices of bacon.