Heat a large skillet over medium heat and then add olive oil. When oil is warmed, add onion and sauté for 3 to 5 minutes, or until nicely softened. Add garlic and cook for 1 minute more. Stir in tomato paste and cook for another minute, until fragrant. Stir in diced tomatoes and red pepper flakes, and bring to a low boil.
Reduce heat to medium low, stir in white balsamic vinegar (if you use regular balsamic vinegar, your dip will be a darker color, and not as bright and vibrant), and simmer for 10 minutes. The dip should be very fragrant and slightly thickened. Season to taste with salt. If you like a smoother dip, pulse all or a portion of the mixture in a blender.
Serve warm with grilled cheese sandwiches for dipping.
Nutrition information is automatically calculated by Spoonacular. I am not a nutritionist and cannot guarantee accuracy. If your health depends on nutrition information, please calculate again with your own favorite calculator.