Zest one of the limes and set aside. This zest will be used later but you want to do this before juicing the limes. Afterwards really isn't an option.
Now add the 4 teaspoons of serrano pepper and the 2 limes, juiced, to a small container and set it aside. Let it infuse depending on how hot you want the dressing. About 5 minutes should give you a decent amount of heat and body. If you want more, let it set for longer.
While the serrano is infusing the lime juice, mix together the remaining ingredients. Just add the lime zest, 4 tablespoons of olive oil, 1/4 cup cilantro, 3/4 teaspoon salt and 1/2 teaspoon sugar to a small mixing bowl and whisk it all together.
Now add the lime juice, straining out the serranos. Mix together and enjoy!