Grease a 2L casserole dish. Cover bottom with half the macaroni. Cover with half the cheese and all the sun-dried tomatoes. Top with rest of macaroni and finish with layer of cheese. In a separate bowl, whisk together 1 1/2 cups milk, egg, mustard and pepper. Pour over mac-n-cheese. Gently push down on casserole with a fork; if you can’t see milk seeping through, add another 1/2 cup of milk. Bake, covered, at 350°F for 35-45 min. Remove from oven and let stand 10 min. before serving.