In a small bowl, toss together avocado and lemon juice. Set aside.
Place tomatoes in a medium bowl, and sprinkle with ¼ teaspoon salt. Add shrimp, onion, vinegar, oil, pepper, and remaining ½ teaspoon salt, stirring to combine. Gently stir in avocado mixture.
Spread goat cheese on crostini. Top with tomato mixture. Garnish with parsley, if desired. Serve immediately.
Recipe by Louisiana Cookin' at https://louisianacookin.com/11510-2/