Add the vinegar, sugar, red onion, mustard powder and salt to a blender. Cover and puree on high until smooth.
Keep the blender processing on low and slowly drizzle in the oil. Blend until incorporated and the dressing is creamy. Stir in the poppy seeds; set aside.
ASSEMBLE THE SALAD:
In a large bowl, toss the lettuce with the bacon, oranges, cranberries and enough dressing to lightly dress the salad.