In a bowl, whisk together olive oil, orange zest, orange juice, finely chopped onion, rosemary, salt, and pepper until well combined.

Add the asparagus to the bowl and toss to coat. Let the asparagus marinate for 1 hour.

Preheat the grill to medium-high heat for direct cooking.

Place the asparagus on the grill and cook for 10-12 minutes, turning occasionally until they are tender and slightly charred.

Transfer the grilled asparagus to a serving platter and brush them with the remaining marinade before serving.