Prepare the Lamb Ribs
1. Preheat oven to 300 degrees Fahrenheit (150 degrees Celsius).
2. Place the lamb riblets in a nine x 13-inch (20×30-centimeter) baking pan. Combine onion salt, paprika, maple syrup, and tomato paste in a small bowl and smear all over the lamb.
3. Pour broth into the pan and cover tightly. Bake for two and a half hours.
Prepare the Pesto
1. Preheat oven to 350 degrees Fahrenheit (175 degrees Celsius). Roast pistachios for 10 minutes, or until fragrant. Let cool.
2. In a food processor, pulse pistachios until finely ground (you can also leave some larger pieces for texture). Add lime juice, parsley, and salt and pulse to combine.
3. Pour in olive oil in a thin stream until the consistency is such that you can dip into it with a spoon, but it is not too thin. Serve over or alongside lamb.
If you like your food with a kick, add a jalapeño to the pesto mixture when you add the lime juice, parsley, and salt.
Lamb riblets are very fatty. For the best yield, choose a cut with as much meat as you can find. If you want something leaner, this spice mix and pesto combo would work great on lamb chops, too.
Prep ahead: The pesto can be made up to two days in advance.