Heat 1 tablespoon of the oil in a large pot over medium heat. Add the onion, ½ teaspoon salt, and ¼ teaspoon pepper and cook, stirring occasionally, until tender, 6 to 8 minutes.
Add the collard greens, tomatoes (with their juices), and ½ cup water. Cover and cook, tossing occasionally, until the greens are tender, 18 to 20 minutes (if the pan becomes dry, add up to ½ cup water).
Meanwhile, heat 1 tablespoon of the remaining oil in a large nonstick skillet over medium-high heat. Season the fish with the Cajun seasoning and ¼ teaspoon salt. Cook, in 2 batches, until opaque throughout, 3 to 4 minutes per side.
Heat the black-eyed peas with 2 tablespoons water and the remaining tablespoon of oil in a small pot over medium heat until warmed through, 2 to 4 minutes.
Serve the fish with the collard greens, black-eyed peas, and lemon wedges.
Way too spicy for our family...