Spray the Romanesco with the olive oil and season it with salt and pepper. Place it on a baking pan and place it under the broiler. Cook it until lightly golden brown, about 6- 8 minutes. Remove from the broiler and let cool.
Once cooled, cut the Romanesco into bite size pieces. Place in a mixing bowl, add the bell peppers and onions and toss.
In another bowl, whisk together the dill, vinegar, lemon juice and mustard. Toss with the contents of the other bowl and season with salt and pepper. Serve.