Stir together yoghurt, lemon juice, and salt in a medium bowl until well combined. Add cucumber, stirring to dress cucumber in yogurt mixture.
Wrap steaks in paper towels, and let stand at room temperature 1 hour. Meanwhile, bring a large pot of water to a boil over medium-high. Fill a large bowl with ice water; set aside. Add okra to boiling water, and cook until bright green and barely tender, 2 to 3 minutes. Using a spider or slotted spoon, transfer okra to ice water, and let cool 2 minutes; remove from ice water, and pat dry using paper towels. Add onions to boiling water, and cook until skins begin to loosen, about 1 minute. Transfer onions to ice water, and let cool 2 minutes. Remove onions from ice water, and carefully trim stem and root ends; use fingers to rub and peel off skins.
Stir together butter and 2 1/2 tablespoons suya spice mix in a small bowl; set suya butter aside.
Unwrap steaks, and sprinkle all sides evenly with 4 teaspoons salt; let stand 20 minutes. Meanwhile, skewer okra and onions separately, spaced 1cm apart, on flat metal skewers. Brush okra and onions all over with oil, and sprinkle evenly with remaining 3/4 teaspoon salt.
Open bottom vent of a charcoal grill completely. Light charcoal chimney starter filled with briquettes. When briquettes are covered with gray ash, pour them onto bottom grate of grill, and push to one side of grill. Scatter applewood chunks over hot coals. Coat top grate with oil; place on grill. Place steaks on oiled grate over side without coals. Grill, uncovered, flipping steaks every 5 minutes, until a thermometer inserted in thickest portion registers 50°C, 25 to 30 minutes. Transfer steaks to grate directly over coals; grill, uncovered, until a thermometer inserted in thickest portion registers 60°C (for medium-rare), about 2 minutes per side, or to desired degree of doneness, smearing top side of each steak with 1 tablespoon suya butter during final 1 minute of grilling time. Transfer steaks to a cutting board, and let rest 10 minutes. Meanwhile, place okra and onion skewers on grates directly over coals; grill, uncovered, until tender and charred in spots, about 2 minutes per side. Remove from grill.
Remove okra and onions from skewers, and transfer to a platter. Remove bones from steaks; thinly slice steaks against the grain into 1/4-inch-thick slices. Arrange steak slices and bones on a platter with okra and onions. Sprinkle okra and onions evenly with remaining 1/2 tablespoon suya spice mix (or more to taste). Serve steak and vegetables with remaining suya butter and cucumber-yogurt sauce.