Heat a skillet with about an of inch water over medium-high heat. When the water boils, reduce heat to a low simmer and add hot dogs to gently heat through.
Heat an outdoor grill or griddle pan over medium-heat or preheat oven to 375°F.
Heat a medium skillet over medium-high heat with the oil, a turn of the pan. Add onions, garlic and Serrano peppers, and season with salt and pepper. Cook to soften a couple of minutes, stirring constantly. Add beans and water, and stir to combine; season with cumin.
Slather the tortillas with beans and top with cheese. Shake excess water from the dogs, place them on one end of tortillas then wrap and roll them. Spray flautas with cooking spray or lightly brush with oil and grill, griddle or bake until dogs are crispy at ends and the tortillas are golden and crunchy-looking.
If making the Pico de Gallo, combine red onions, Serrano or jalapeno and salt, and let sit a few minutes. Add in tomatoes and cilantro.
Stir together the Crema and mustard.
Place flautas on a bed of lettuce or scatter lettuce over them. Top with pico de gallo and a few pickled jalapeños, some yellow mustard crema and diced avocado dressed with lime.