Combine the tuna with the lime juice mixture just before serving to maintain the bright red color of the fish--a must-do if you like tuna rare. If you want "cooked" tuna, soak it in 1/2 cup lime juice (enough to cover) for 1 to 3 hours, stirring occasionally, until opaque. (The fish will turn a grayish brown.) Drain, and stir in remaining ingredients.
How to Make It
Soak noodles according to package directions; drain. Rinse with cold water; drain. Cut into 3/4-inch-long pieces using scissors.
Combine lime juice and next 6 ingredients in a large glass or ceramic bowl. Stir in noodles, cilantro, green onion, avocado, and cucumber. Cover and refrigerate at least 30 minutes. Top noodle mixture with tuna; stir tuna into noodle mixture just before serving. Sprinkle evenly with sesame seeds.
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