In large saucepan or Dutch oven, cook fettuccine to desired doneness as directed on package.
Meanwhile, heat oil in large nonstick skillet over medium heat until hot. Add chicken; cook and stir until no longer pink. Add mushrooms, shallots and 1 to 2 teaspoons marjoram; cook until thoroughly heated. Stir in salt and pepper.
Drain fettuccine; return to saucepan. Stir in spinach and hot chicken mixture. Cook over low heat until thoroughly heated, stirring occasionally. Stir in chopped tomatoes. Spoon onto large serving platter. Garnish with tomato slices and fresh marjoram.