SALSA: In a bowl, combine the chopped onions, tomatoes, jalapenos, and drizzle with the jalapeño juice. Add the kosher salt, stir to combine and taste. Adjust with more salt as needed.
SLICE: Slash, scale, crosshatch, or spiral cut the hot dogs, if desired. You can easily skip this step as well.
PEPPERS: Heat a cast iron or nonstick grill pan (or skillet) over medium high heat. Spray the pan with cooking spray, or add a tablespoon of butter. Place the sliced Serranos on the grill pan on one side.
HOT DOGS: Add the hot dogs in a single layer on the other side and cook all the ingredients for 3-5 minutes, flipping every 45-60 seconds or as needed to brown the hot dogs evenly and to char the serrano peppers. Remove the Serranos as they char to a plate.
SERVE: Spread the mayonnaise on the toasted (or steamed) buns. Then nestle the hot dogs inside the buns. Top with the prepared salsa, drizzle with ketchup and mustard as desired. Add the sliced, charred peppers and top with a sprinkle of cilantro for color. Serve warm.