Bring a large pot of water to a boil. Add the macaroni and cook until al dente. Drain.

Meanwhile, combine the half and half and the milk in a saucepan and set over medium heat. Heat until warmed through, being careful not to let it come to a boil.

In another large saucepan, melt the butter. Whisk in the flour and cook for 1 minute. Slowly whisk in the hot milk and cook until thickened, about 5 minutes. Stir in the gruyere and the mozzarella. Season to taste with salt and pepper and add in the ground mustard. Add the cooked macaroni to the cheese sauce and stir to combine. Pour the macaroni and cheese into a dish to cool, or refrigerate overnight.

When the macaroni and cheese has completely cooled, place a large, heavy duty pan or Dutch oven over medium heat. Add several inches of vegetable oil and heat to 350F.

Working with one egg roll wrapper at a time, moisten the edges with water. Place the wrapper with one point facing toward you. Place about 1/4 cup of the macaroni and cheese in the center and roll the corner closest to you over the filling. Fold in the sides of the wrapper and continue rolling until it is closed. Press to seal and set aside. Continue with the remaining wrappers.

Place the egg rolls in the heated oil and fry, turning occasionally, until golden brown. Remove and drain on paper towels or a wire rack. Serve warm.