In a large skillet, heat 2 tablespoons of extra-virgin olive oil over medium heat.
Add the chopped white onion to the skillet and sauté for about 5 minutes or until it becomes translucent. Then, add the chopped garlic and cook for an additional minute, stirring frequently to avoid burning.
Stir in the chopped roasted red bell peppers and kosher salt. Cook for another 5 minutes, allowing the flavors to meld together.
Remove the skillet from heat and let the mixture cool slightly. Transfer it to a blender or food processor and blend until smooth.
Add the heavy cream, chopped fresh basil leaves, and balsamic vinegar to the blended mixture. Blend again until everything is well-combined and creamy.
If the coulis is too thick, add vegetable stock a tablespoon at a time until you reach your desired consistency. Blend once more to mix.
Taste your Red Pepper Coulis and adjust seasoning with more salt or vinegar if necessary.