Preheat oven to 375°F. Steam or boil beans until just tender, 5-6 minutes, then drain.
Melt butter in a skillet over medium heat. Cook onion 5 minutes until soft, then stir in garlic 30 seconds.
Whisk mustard, honey, sour cream, and broth in a bowl. Season with salt and pepper.
Toss beans, onion mixture, and sauce in a baking dish. Stir in cheddar and half the pecans.
Mix panko with olive oil and sprinkle over casserole. Top with remaining pecans.
Bake 20-25 minutes until bubbly and golden on top. Let sit 5 minutes before serving.