Mix 1/3 c pineapple juice, soy sauce, mustard, garlic, ginger and brown sugar.
Cut fish into 4-5 servings and place in a pan.
Pour marinade over fish let stand 20 min turn over and marinate 20 more minute.
Heat grill or broiler in oven. Remove fish from marinade, reserving marinade. Spray fish with non-stick spray; grill or broil 3-4 minutes per side (about 10 minutes total for every 1-inch thick fillets). Grill pineapple slices at the same time fish is grilling.
Place marinade in a small saucepan boil then remove from heat.
After cool wisk in vinegar and oil.
Place washed spinach on plates, arrange tomatoes and crumbled feta over spinach. Place grilled fish in middle of plate and drizzle with 1-3 tablespoons dressing.