1. In a small bowl, stir together butter, miso, chives, and garlic. Spoon butter mixture onto a square of plastic wrap, fold plastic over butter from the top and the bottom, and use your hands to form it into a log shape about 1 1/2 in. thick. Twist sides to close. Put in freezer for 30 minutes or until firm.
2. Meanwhile, rinse steaks, pat dry, and season well with salt and pepper; set aside at room temperature, 15 to 25 minutes. Peel onions and cut in half crosswise. Trim about 1/2 in. off rounded sides of each onion so halves lie flat. Rub onions with some olive oil and salt and pepper to taste.
3. Prepare a gas or charcoal grill for medium-high heat (you can hold your hand 1 to 2 in. above cooking grate for only 3 to 4 seconds). Grill onions 2 minutes, then add steaks and grill until nicely browned, 3 to 4 minutes. Turn everything over. Onions will need another 6 to 8 minutes, until they're softened and nicely browned. For steaks, cook an additional 2 to 4 minutes for rare, 5 to 7 minutes for medium-rare, and 8 to 15 minutes for well done (cut to check).
4. Top each steak with 2 tsp. miso butter (you will have some butter left over). Serve with onions and a simple green salad if you like.
Note: Nutritional analysis is per serving.
Find miso—fermented soybean paste—in Asian markets, natural-foods stores, and supermarkets.