How to Make It
Combine mustard, coriander, onion powder, garlic powder, cayenne pepper, black pepper, and 1/8 teaspoon salt in a bowl. Pat scallops dry with a paper towel; sprinkle with spice mixture.
Heat a large cast-iron skillet over medium-high. Add 2 teaspoons olive oil to skillet; swirl to coat. Add scallops; cook to desired degree of doneness, 1 to 2 minutes per side.
Toss together watermelon, onion, 2 tablespoons mint, 1 tablespoon lime juice, 2 teaspoons olive oil, and remaining 1/8 teaspoon salt in a bowl. Combine quinoa, pine nuts, scallions, remaining 2 teaspoons olive oil, remaining 2 tablespoons mint, and remaining 1 tablespoon lime juice. Serve scallops over quinoa mixture; top with salsa.