How to Make It
In a small saucepan, heat the buttermilk, water and sugar until an instant-read thermometer registers 95°. Transfer to the bowl of a stand mixer and whisk in the sourdough starter and yeast. Let stand until foamy, about 20 minutes. Attach the dough hook. Add the 3 3/4 cups of bread flour and mix on low speed until smooth, about 7 minutes. Add 2 1/2 teaspoons of salt and mix on low speed until incorporated, about 3 minutes. Transfer the dough to a large greased bowl, cover with plastic wrap and let rise in a warm place until doubled in bulk, about 45 minutes.
Cut the dough into 20 pieces. Gently shape into balls and transfer to 2 large greased baking sheets. Very loosely cover the dough with plastic wrap. Let stand in a warm place until doubled in bulk, about 2 hours.
Heat a cast-iron griddle or large skillet over moderately high heat. On a lightly floured work surface, dust 1 ball of dough with flour and roll 1/8 inch thick. Griddle the tortilla, turning once, until browned in spots and pliable, about 30 seconds per side. Wrap the tortilla in a towel and repeat with the remaining balls of dough.
Light a grill or preheat a grill pan. Grill the scallions over moderately high heat, turning, until lightly charred, 3 minutes total. Transfer to a work surface to cool, then cut into 1/2-inch pieces. In a medium bowl, mix the scallions with the lime juice and serrano and season with salt. Leave the grill on.
Season the steak with salt and pepper. Grill over moderately high heat, turning once, until lightly charred outside and medium-rare within, about 6 minutes total. Transfer to a carving board and let rest for 5 minutes, then carve against the grain. Serve the steak in the tortillas with the scallion salsa.
Fresh sourdough starter kits are available at kingarthur flour.com. Follow the feeding instructions before using.