Shrimp Vuelve a la Vida

Name: Shrimp Vuelve a la Vida
Ingredients: 1 medium red onion; 3 garlic cloves, 2 smashed, 1 finely grated; 1 Tbsp. black peppercorns; 1 Tbsp. Diamond Crystal or 1¾ tsp. Morton kosher salt, plus more; 1 lb. large shrimp, peeled, deveined; 1 plum tomato, coarsely chopped; ½ cup coarsely chopped cilantro stems; 1 cup fresh lime juice (from about 8 limes); 1 cup ketchup; 1 Tbsp. plus 2 tsp. distilled white vinegar; 1 Tbsp. plus 2 tsp. Tabasco; 1 Tbsp. Worcestershire sauce; 2 tsp. Aji-No-Moto umami seasoning (MSG; optional); 2 tsp. sugar; 1 large ripe avocado, thinly sliced; Cilantro leaves with tender stems, saltine crackers or corn tostadas, and lime wedges (for serving)