Roast the tomatoes on a rimmed baking sheet about 4 inches below a preheated broiler until soft, blackened and blistered, 5 or 6 minutes per side. Remove and let cool. Meanwhile, turn the oven to 425 degrees. On another baking sheet, drizzle the onion and garlic with the oil and toss to combine. Roast, stirring a few times, until the richly browned, about 15 minutes total. When the tomatoes are cool, pull off their blackened skins.
Scoop the tomatoes into a food processor and pulse several times to chop them into small pieces. Scrape into a large bowl. Without washing the food processor, scoop in the onion and garlic, then pulse until coarsely chopped. Scrape in with the tomato and stir in the vinegar, brown sugar and hot sauce. Taste and season highly with salt, usually about 1 1/2 teaspoon.
If you're a person that likes shrimp cocktail really cold, refrigerate the sauce until chilled. Add the shrimp, avocado and cilantro to the sauce and stir to combine. Coctel de Camaron looks beautiful served in martini glasses or little glass bowls (especially if they are nestled into a larger bowls filled with crushed ice).