Choose 6 fish fillets the same size or cut ones into 6 serving-size pieces. Heat oven to 350 degrees F. Spread oil in 13x9x2-inch baking dish. Seperate onion slices into rings; spread them plus pimiento on bottom of pan. Lay fillets on top; sprinkle with salt, pepper, cayenne and mace; top each with tomato. Sprinkle green onions and mushrooms over all; drizzle wiht lemon juice. Combine crumbs and butter; sprinkle on top. Bake, uncover, till fish flakes easily with fork, about 30 minutes.