Combine sour cream, horseradish, and 1/4 teaspoon salt; stir well with a whisk. Set aside.
Divide meat into 4 equal portions, shaping each into a 1/2-inch-thick patty.
Place patties in a nonstick skillet over medium-high heat. Sprinkle with 1/2 teaspoon Cajun seasoning. Cook 5 minutes; turn, sprinkle with 1/2 teaspoon Cajun seasoning, and cook 4 minutes. Remove patties from pan, and keep warm.
Coat pan with cooking spray, and place over medium-high heat. Add bell pepper, 1/4 teaspoon salt, onion, and squash. Cover, reduce heat to medium-low, and cook 6 minutes or until squash is crisp-tender, stirring occasionally.
Return patties to pan; add water. Cover and cook over medium heat 3 minutes or until done. Serve with sour cream mixture.