Preheat the grill to medium-high heat (about 400°F) or preheat the oven to 425°F (220°C).
In a large bowl, toss the steak cubes, quartered potatoes, sliced bell peppers, and onion with olive oil, garlic powder, onion powder, smoked paprika, salt, and pepper until evenly coated.
Divide the mixture evenly among 4 large sheets of heavy-duty aluminum foil (about 18x12 inches each). Top each portion with a pat of butter and a few sprigs of fresh thyme and rosemary.
Fold the foil into tightly sealed packets, leaving a little room at the top for steam to circulate.
Grill for 18 to 20 minutes, or bake in the oven for 25 minutes, until the potatoes are fork-tender and the steak reaches your desired doneness.
Let the packets rest for 5 minutes before carefully opening (watch for hot steam). Serve directly from the foil.
Notes
Use heavy-duty foil to prevent tearing. Cut the potatoes small so they cook at the same rate as the steak. Do not overcrowd the packets or nothing will cook evenly.