Make the marinade: In a blender, add the mint, garlic, lime juice, sugar, salt, and roasted cumin; purée on medium speed. (Add 1–2 tablespoons of water if needed to help all the ingredients come together in the blender.) With the machine running on low speed, slowly drizzle in the grapeseed oil until it is completely incorporated. Pour ⅓ cup of the marinade into a small bowl, cover, and refrigerate. Set the remaining marinade aside.
Make the kebabs: In a large bowl, add the fish and season lightly with salt. Pour the remaining marinade over the fish and toss well to coat. Cover loosely with plastic wrap and refrigerate for at least 10 minutes while you preheat the grill. (Do not marinate longer than 30 minutes, or the lime juice will begin to "cook" the fish.)
Meanwhile, soak ten 8-inch bamboo skewers in cold water for at least 30 minutes before grilling.
Preheat a grill to high (about 450°F) or a grill pan over very high heat. Drain the skewers and shake off any excess water. Thread 3–4 chunks of fish onto each skewer, leaving a slight gap between each chunk
Transfer the kebabs to the grill and cook with the grill open until lightly charred on one side, about 2–3 minutes. Using a fish spatula or a thin metal grilling spatula, loosen and turn the kebabs. (If the fish is sticking, give it another minute or so. It should release easily once it's fully cooked on one side.) Continue cooking until the opposite side is lightly charred and the fish is cooked through, 2–3 minutes more. Transfer to a platter, drizzle the remaining marinade on top, and garnish with dill, sumac, and a drizzle of olive oil. Serve hot or at room temperature.