Soak the bread in water for 10 minutes; squeeze dry and pulse in a food processor with the walnuts, grated onion, 1 teaspoon paprika, cinnamon, mint, cumin, cayenne, 1 1/2 teaspoons salt and 1/2 teaspoon black pepper. Add 1 tablespoon dill and the parsley; pulse until smooth.
Knead the spice paste and beef in a bowl. Form into 8 egg-shaped pieces, then flatten into 1/2-inch-thick oval patties. Refrigerate for 10 minutes. Meanwhile, soak the sliced red onion in ice water; drain.
Toss the cantaloupe, lemon juice, 1 teaspoon salt, onion slices and the remaining 1 tablespoon dill and 1/2 teaspoon paprika in a bowl.
Preheat the grill to medium-high. Brush the patties with olive oil. Grill about 5 minutes; flip and continue grilling until cooked through but still moist, 4 more minutes. Serve with the melon slaw.