Position a wire rack on the top level of the oven, about 6 inches from the broiler. Preheat broiler on high.
On a cookie sheet, place onion slices and tomatoes cut side down. Roast until a bit charred on the edges, about 10 minutes.
Over medium heat, sauté garlic in 1 tablespoon of vegetable, turning several times with tongs, about 1 minute. Add chiles to garlic, sauté for 1-2 minutes, flipping occasionally until they just start to toast. Take care not to burn.
Transfer chiles (including habaneros) and garlic to a high-speed blender. Pour in the water and vinegar, and let stand for 5 minutes to soften.
Purée on high, 10 to 20 seconds. Turn to low speed and slowly add remaining vegetable oil in a slow and steady stream so the oil mixture will emulsify, creating desired creamy consistency.
Add the tomatoes, onion and salt. Purée on high, until smooth.
Serve now or store in the fridge for about 2 weeks. Enjoy!
