Open can pineapple and drain juice into a cup. Combine 1/3 cup pineapple juice with soy sauce, dry mustard, garlic, ginger and brown sugar; stir to blend. Cut salmon into 4-5 servings and place in shallow dish. Pour marinade over fish and to stand 15-20 minutes; turn once and allow to marinade an additional 15-20 minutes.
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 fillet). Grill pineapple slices at the same time fish is grilling.
While fish is grilling, place marinade in small saucepan and bring to a full boil. Remove from heat, cool, and whisk in vinegar and oil.
To serve: 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.