Step 1

Make the two-onion quick pickle: In a medium bowl, combine the scallions and half of the onion. Cover the onions and scallions with hot tap water and squeeze them a few times with your hands. Let stand for about 2 minutes to extract their raw flavor, then drain. (This will lessen your onion breath.)

Step 2

In a small jar or liquid measuring cup, combine the lime juice, the rinsed onions and scallions and a pinch of salt. Let stand while you make the remainder of the recipe, or for at least 10 minutes.

Step 3

In a large skillet over medium heat, heat the oil until shimmering. Add the remaining onion and cook without stirring until lightly browned, about 3 minutes. Add the black beans with their liquid, chili powder, cumin, Spanish smoked paprika, Dijon mustard and 1/4 teaspoon of salt and stir to combine. Cook, stirring occasionally, until the liquid reduces to a thick and fragrant sauce, 5 to 7 minutes.

Step 4

We like to add a bit of flavor and lovely char marks while warming our tortillas (not required, but highly recommended). Place a tortilla right on the grate above the flame of a gas burner, then cook on medium heat for a few seconds per side, flipping with tongs, until lightly blackened on the edges. Repeat with the remaining tortillas. (If you don’t have a gas range, arrange the tortillas in a single layer on a baking sheet and broil for about 1 minute per side.)

Step 5

To assemble the tacos, place some chopped romaine at the center of each tortilla, then add a large spoonful of the black bean mixture. Top with the pickled onion and hot sauce, if using. If desired, you can pour a small splash of the lime juice from the onion pickle onto each taco (but be conservative, or you’ll have a watery taco).