Reserve 1 teaspoon dressing mix. Mix together sour cream and remaining dressing mix in a medium bowl. Whisk in eggs and milk.
Divide baguette halves between two 13 x 9 x 2-inch baking dishes. Place crust side up. Pour egg mixture evenly over bread in each baking dish, about 1½ ; cups egg mixture in each baking dish. Cover, refrigerate overnight.
Remove baking dishes from refrigerator. Place baguettes on a tray. Discard any excess liquid from dishes. Wash and wipe out baking dishes. Spray bottoms of baking dishes with nonstick spray. Place baguettes back in dishes, crust side down. Turn oven to 350ºF. Place dishes in oven. Bake 35-40 minutes, or until tops are puffed and lightly golden.
Toss together tomatoes and onions with reserved one teaspoon of salad dressing mix in a shallow bowl or pie plate. If desired, season with salt and pepper. In a small bowl, mash together avocado and mayonnaise creating a chunky spread.
Once French toast is removed from oven, transfer baguette bottoms to a large platter. Spread some of the avocado spread over bottoms. Assemble as follows: lettuce, bacon, tomato mixture. Divide remaining avocado spread between baguette tops. Place baguette halves together. Cut in quarters. Serve warm.