How to Make It
Heat a large (12-inch) cast-iron skillet over medium-high. Add cherry tomatoes, and cook, stirring often, until skins begin to split and char, 5 to 7 minutes. Add chopped onions and minced garlic, and cook, stirring often, 5 minutes. Add butter to pan, and cook until melted, about 1 minute. Stir in brown sugar. Cook, without stirring, until edges start to caramelize, 2 to 3 minutes.
Stir in vinegar, mustard, Worcestershire sauce, salt, and pepper; reduce heat to medium-low, and cook until tomatoes break down and mixture is slightly thickened, about 45 minutes.
Remove from heat; carefully transfer to a blender. Remove center piece of blender lid (to allow steam to escape); process on high until almost smooth, about 1 minute. Cool completely, about 30 minutes. Store in an airtight container in refrigerator up to 2 weeks.