Preheat oven to 350 degrees F.
Combine 1 Tbsp oil and garlic in small bowl. Brush garlic-oil mixture over butternut squash pieces, and place on baking sheet.
Sprinkle each squash piece with 1/2 tsp Cajun seasoning, and season with salt and pepper, if desired.
Bake 45 minutes to 1 hour, or until squash pieces are tender enough to easily pierce with fork.
Meanwhile, heat remaining 1/4 cup oil in large skillet over medium heat. Add onions, bell peppers, and celery, and cook 20 to 25 minutes, or until vegetables are soft and beginning to brown, stirring occasionally.
Stir flour into onion mixture, and cook 1 minute, or until flour begins to brown, stirring constantly. Stir in vinegar, remaining 2 Tbsp Cajun seasoning, and 6 cups water. Season with salt and pepper, if desired.
Reduce heat to medium-low, and simmer 20 minutes, or until sauce has thickened, stirring occasionally.
When ready to serve, place 2 squash pieces on plate. "Smother" with 1 cup onion mixture.