In a large Dutch oven, bring 6 cups water, peas, and 1 teaspoon salt to a boil over medium-high heat; reduce heat to medium-low, and simmer, stirring occasionally, until almost tender, about 20 minutes.
Add beans to peas mixture; simmer, stirring occasionally, until peas are tender and beans are crisp-tender, about 7 minutes. Drain, and transfer to an ice water bath to stop the cooking process. Pat dry with paper towels.
In a large bowl, whisk together lemon zest and juice, dill, sugar, pepper, garlic, and remaining ¾ teaspoon salt. Whisk in olive oil in a slow, steady stream. Add peas mixture, radish, and onion, tossing to coat. Cover and refrigerate for 1 hour, tossing occasionally.
For fish:
Brush a cast-iron grill pan with vegetable oil; heat over medium heat.
Sprinkle salt and Cajun seasoning all over fish. Add fish to pan, and cook until it flakes easily with a fork, about 4 minutes per side.
Toss salad, and top with fish. Top with cheese and mint.
Recipe by Louisiana Cookin' at https://louisianacookin.com/chilled-pea-bean-salad-with-snapper/