Charred scallions and fresh mint come together in a smoky-sweet relish that elevates this quick pork dish.
Heat grill to high. Season pork with salt and pepper. Stir together mustard, fennel seeds, and 1 teaspoon oil; rub mixture all over pork. Cook couscous in a pot of boiling salted water until al dente, 5 to 7 minutes. Drain; toss with 1 tablespoon oil. Drizzle 2 teaspoons oil over vegetables; season with salt and pepper.
Brush grates with oil. Grill vegetables, flipping once, until tender and charred in places, about 3 minutes for scallions and 6 minutes for squashes. Grill pork, turning occasionally, until browned on all sides and a thermometer inserted into thickest part registers 138 degrees, 12 to 15 minutes. Whisk together vinegar, remaining 4 tablespoons oil, and mint. Season with salt and pepper. Chop scallions; stir into mint mixture. Toss couscous with squashes and 1/2 cup scallion relish. Slice pork and serve, with couscous and remaining relish.