Adjust rack to middle position (top and middle positions for 4 servings) and preheat oven to 425 degrees. Place 2 TBSP butter (3 TBSP for 4) in a small bowl and bring to room temperature. • Pat chicken* dry with paper towels and season all over with salt and pepper; place on one side of a baking sheet (spread out across entire sheet for 4). • Brush tops of chicken with ½ TBSP honey Dijon dressing (save the rest for serving). Sprinkle with Monterey Jack. Crush crispy onions in their package, then sprinkle over cheese, pressing to adhere. • Roast on middle rack for 5 minutes (you’ll add more to the sheet then).
Meanwhile, wash and dry produce. Trim green beans if needed. • Once chicken has roasted 5 minutes, remove sheet from oven. Carefully toss green beans on empty side with a drizzle of oil; season with salt and pepper. (For 4 servings, leave chicken roasting; add green beans to a second baking sheet and roast on top rack.) • Return to middle rack and bake until green beans are tender and chicken is browned and cooked through, 12-15 minutes more.
Meanwhile, combine softened butter with garlic powder to taste. Season with salt and pepper. • Halve and toast ciabatta. Spread cut sides with garlic butter. Cut each half on a diagonal.
Toss green beans with 1 TBSP plain butter (2 TBSP for 4 servings) until melted. Season with salt and pepper. • Divide chicken, green beans, and garlic bread between plates. Serve with remaining dressing on the side.