Make sauce: Preheat oven to 425°Heat 1 tablespoon oil in a medium saucepan over medium. Add taco seasoning, 2 tablespoons of tomato paste (save rest for own usand 1 tablespoon flour; cook, whisking for 1 minute. Slowly whisk in 1¼ cups water and broth concentrate; bring to a simmer. Cook until slightly thickened, about 10 minutes. Season with ½ teaspoon salt.
Prep ingredients: Grate cheese on large holes of a box grater. Trim ends from scallions then thinly slice, keeping dark greens separate. Rinse and drain black beans. Finely chop spinach. Finely chop 1 tablespoon pickled jalapeños.
Assemble the filling: In a large bowl, combine beans, corn, scallion whites, spinach, chopped jalapeños, and ⅔ of the cheese. Season with ¾ teaspoon salt and a few grinds pepper, and stir to combine.
Heat tortillas: Brush tortillas on both sides with oil, place on a rimmed baking sheet (overlapping is fine). Bake tortillas on the center oven rack until just warmed and softened, about 2 minutes (watch closely). Wrap in foil or a clean kitchen towel to prevent from drying out, keep covered until ready to use.
Assemble enchiladas: Lightly oil the inside of a medium baking dish or ovenproof skillet. Pour ¼ cup sauce in baking dish and spread to coat the bottom. Arrange tortillas on a work surface. Divide filling evenly among tortillas (about ⅓ cup each); roll up tightly and arrange, seam side-down, in the baking dish. Top with remaining sauce.
Bake enchiladas & serve: Sprinkle enchiladas with remaining cheese and bake on center oven rack until browned and bubbling, 18–22 minutes. Sprinkle scallion greens on top and serve with remaining jalapeños on the side.