Combine shallot and vinegar in bowl and let sit for 20 minutes.
Using fork, remove shallot from vinegar; reserve shallot and 2 tablespoons vinegar. Whisk 3 ounces blue cheese (about 3/4 cup), mayonnaise, sour cream, milk, reserved vinegar, garlic, salt, and pepper together in bowl until combined.
Hold the head of iceberg lettuce with both hands, with the core pointed toward the counter. Hit the core end of the lettuce forcefully against the counter to loosen the core. Twist and pull the core out of the bottom of the head.
Cut it into 6 wedges. Start by cutting it in half and then each half into 3 equal wedges.
Arrange lettuce wedges on platter and top with dressing, reserved shallot, and tomatoes. Crumble bacon over top and sprinkle with remaining 1 ounce blue cheese.