Brush fish with olive oil and season with salt and pepper.
Prepare fish on grill, under broiler or in a heavy skillet.
Cook 3 to 4 minutes per side until done. Place in refrigerator tocool. When cool, cut into bite-sized pieces and add to cooled Curry Sauce.
Split croissants and lightly butter inside. On grill or in a skillet, lightly brown buttered sides. Assemble sandwiches.
For sauce: Sauté onion and garlic in olive oil until tender. Add curry powder and cook 2 to 3 minutes.
Add water, and incorporate into mixture. Remove from heat; add mayonnaise, lemon juice, and salt and pepper. Cool until ready to use. May be made ahead.