In a bowl, mix together all the dressing ingredients. Depending on how thick your mayonnaise is, you may need to add a little bit of the vinegar at a time to make sure it doesn't get too runny. Best Foods mayo is very thick and I found the 2 tsp of vinegar made it the perfect consistency.
Keep any leftover dressing in a covered jar in the fridge for a week or two.
Assemble your salad in a large bowl. Use the cos lettuce as a base and then add the toppings on top. When you're ready to serve drizzle with the dressing to taste and toss.