Preheat broiler to high. Line baking sheet with aluminum foil.
Place tomatoes, onions, jalapeños, and garlic on baking sheet. Drizzle neutral oil over all and toss to coat thoroughly, then spread ingredients out in single, even layer.
Place baking sheet 6 inches below broiler. Broil 15 to 20 minutes, stopping to flip vegetables over halfway through cook time. Cook until skins of veggies have thoroughly blistered and blackened.
Optional: transfer blackened onion and garlic to plate and set aside. Return tomatoes and jalapeños to broiler for another 5 minutes for maximum smokiness.
Remove all vegetables from oven. Carefully remove stems and seeds from jalapeños, leaving seeds from 1 jalapeño for medium spiciness, or no seeds at all for mild spiciness.
Place roma tomatoes, onion, jalapeños, garlic, and cilantro in food processor bowl. Pulse until mostly smooth, then add in drained diced tomatoes. Continue pulsing until desired salsa consistency is achieved.
Transfer salsa to serving bowl or airtight container. Add salt and lime juice to taste, and stir well to thoroughly incorporate. Serve immediately or seal airtight container and refrigerate until ready to serve.