Season the salmon on both sides with salt, pepper and the Cajun seasoning; set aside. (If your Cajun seasoning contains salt, do not season the salmon with additional salt.) Heat 1 tablespoon olive oil in a medium saucepan over medium-high heat. Add the celery and a pinch of salt. Cook, stirring, until the celery starts to soften, about 4 minutes. Stir in 2 sliced scallions and the garlic; cook until softened, about 1 minute. Add the black-eyed peas and chicken broth. Cover and bring to a boil over high heat, then uncover and reduce to a simmer. Mash about 1/4 cup of the beans against the side of the pan with a wooden spoon. Simmer until the broth is thickened, 8 to 10 minutes.
Meanwhile, heat a large nonstick skillet over high heat. Rub the remaining 1 tablespoon olive oil all over the salmon. Add to the skillet, curved-side down; reduce the heat to medium high and cook until blackened on the bottom, 3 to 5 minutes. Flip, reduce the heat to medium and continue cooking until browned on the other side, 2 to 3 minutes for medium rare to medium.
Add the spinach and hot sauce to the beans; stir until wilted, about 30 seconds. Add more salt and pepper, if needed. Divide the beans and salmon among shallow bowls. Top with the remaining sliced scallion.