Rinse 1 red or yellow bell pepper (about 10 oz.); pat dry. Set pepper in a 9-inch pie or cake pan and broil 3 to 4 inches from heat, turning as needed, until charred on all sides, about 15 minutes total. Let cool. With a small, sharp knife, remove and discard skin, stem, and seeds. Coarsely chop pepper and place in a blender or food processor. Add 1 tablespoon water and 1 teaspoon olive oil; whirl until smooth. Add salt and cayenne to taste. If coulis is thicker than desired, thin with 1 to 2 more tablespoons water.