Place Marinade in medium bowl. Reserve 2 tablespoons of the Marinade for the dressing. Brush pineapple and onion rings lightly with some of the remaining Marinade in the bowl. Add tuna into remaining Marinade; toss to coat well. Cover. Refrigerate 15 to 30 minutes. (Marinate seafood no longer than 30 minutes.)
Grill pineapple and onion rings over medium-high heat 3 minutes per side or until slightly tender and charred. Remove from grill. Cool slightly. Meanwhile, remove tuna from marinade. Discard any remaining marinade.
Grill tuna over medium-high heat 1 to 2 minutes per side or until charred on outside but still rare inside. Remove from grill. Let rest about 5 minutes.
Mix rice vinegar, soy sauce, sugar, jalapeño and reserved 2 tablespoons Marinade with whisk in medium bowl. Cut grilled pineapple and onion into 1-inch pieces. Add into dressing mixture; toss to coat. Cut grilled tuna into 1-inch pieces. Divide romaine evenly among 4 salad plates. Top with pineapple mixture and tuna. Drizzle with any remaining dressing.
For a crunchy texture, sprinkle poke with chow mein noodles before serving.