To prepare the roast:
Line a broiler or roasting pan with aluminum foil. Place the beef on the foil and set aside.
Preheat the oven to 325 degrees F.
In a food processor, combine the cilantro, garlic, celery seed, salt, and pepper. Puree until the mixture is a smooth paste. Rub the cilantro-garlic paste all over the roast, covering it completely. Allow the roast to marinate for at least 10 minutes to 1 hour to absorb the flavors.
Roast the beef for about 1 1/2 hours for rare or until an instant-read thermometer registers 125 degrees F. when inserted into the center. Cook for an additional 10 minutes for medium-rare or 20 minutes for medium doneness. Remove the roast from the oven and let rest for 10 minutes before slicing to allow the juices to settle. While the roast rests, make the pesto.
To prepare the pesto:
In a food processor, combine the cilantro, pine nuts, garlic, horseradish, and salt. Puree until well combined. With the motor running, drizzle in 1/4 cup of oil. Process until smooth. Add additional oil, if needed, until a drizzling consistency is reached.
To serve: Using a sharp knife, thinly slice the roast beef. Put a few slices of roast beef on the bottom of each sandwich roll. Drizzle the cilantro pesto directly on the beef.
For picnics, take the pesto along in a plastic container or squeeze bottle and add it just before eating.