Preheat the oven to 375°F. Spray a baking dish with cooking spray.
Boil a large pot of salted water for the pasta. Add the pasta, stirring to prevent clumps. Cook to al dente, according to package instructions, strain and return to the pot, drizzled with oil to prevent sticking.
In a saucepan over medium heat, combine the bechamel, milk, stock and pepper. Bring to a boil, stirring constantly. Turn off the heat and fold in the cheeses, stirring until melted and smooth.
Toss the mushrooms with the olive oil and a pinch of salt and pepper on a baking sheet and roast for 10-15 minutes, until tender.
In the prepared baking dish, combine the pasta, sauce, roasted mushrooms, and corn. Top with breadcrumbs and bake for 10-15 minutes, until golden on top and melty.
For the vinaigrette, combine all ingredients other than the oil in a blender or food processor. Purée until smooth. With the motor running, drizzle in the oil until emulsified. Season with salt and pepper.
Toss the spring mix and tomatoes with the dressing in a large bowl.
Serve the mac and cheese with salad on the side.