Roasting the red bell pepper beforehand will bring out its sweetness and add a depth of flavor. Ensure your Dijon mustard is vegan-friendly, as some brands use wine in their production. The speed and patience with which you add the oil are crucial for a stable emulsion. Add it too quickly, and the mayonnaise may separate. This mayonnaise will keep in the refrigerator for up to one week. For a bolder flavor, try adding a pinch of smoked paprika or garlic powder.