Bring a pot of water to a boil and cook the macaroni according to the package directions. Drain and set aside.
Toss the lettuce with the olive oil and season with salt and pepper. Roast on a baking sheet until the lettuce is wilted, about 15 minutes. Set aside to cool, but leave the oven on.
In a 4-quart saucepan over medium heat, render the bacon with the canola oil until crispy. Remove the bacon and set aside, leaving 3 tablespoons of fat in the pan. Add the flour and cook over medium heat until the flour begins to smell slightly toasted, about 1 minute. Add a small amount of the milk to the flour and whisk until smooth. Add the remaining milk in stages to avoid lumps. Heat until the mixture simmers, then turn off the heat.
Add the Cheddar and Gruyere to the pan, followed by the nutmeg and drained macaroni. Fold until the macaroni is evenly coated, then add the lettuce, tomatoes and bacon.
Spread the mixture in a 2-quart baking dish and top with the Herbed Breadcrumbs. Cook until the top is golden, about 35 minutes.
Yield:About 2 1/4 cups
Combine the panko, butter, parsley and thyme in a medium bowl and mix together.