Bring to a boil and reserve 2 cups to make Citrus Vinaigrette. Add shrimp, garlic, jalapeño, cilantro and red onion. Bring back to a simmer and cook for an additional 2 to 3 minutes. Remove pot from heat.
Pour into a glass or plastic container and refrigerate overnight.
To make Citrus Vinaigrette, combine cornstarch and cold water. Bring 2 cups reserved marinade to boil and whisk in cornstarch mixture. Bring to a simmer until thickened. Cool and reserve.
To serve, fan avocado slices on plates. Place shrimp on top and drizzle with Citrus Vinaigrette.