Combine onion, sun-dried tomato, olive oil, balsamic vinegar, marjoram, sugar, and red pepper flakes in a jar.

Seal the jar tightly and shake well to emulsify the dressing.

Refrigerate the dressing for 1 hour to allow flavors to meld.

In a large bowl, combine red tip leaf lettuce, torn radicchio, cubed cooked chicken, thinly sliced carrots, and sliced mushrooms.

Shake the dressing again to ensure it's well mixed.

Pour the dressing over the salad in the bowl.

Toss the salad lightly to coat all ingredients with the dressing.

Divide the salad evenly onto 4 salad plates.

Sprinkle crumbled blue cheese over each salad plate.