Position an oven rack in the center of the oven and preheat to 375 degrees F.
Remove 1/4 inch of the crust from the ends of the loaf, then cut the loaf crosswise into 4 even pieces. Use a fork to score one cut end and gently dig a square hole into each piece of bread to make a box, leaving a 1/2-inch layer of bread around the walls and bottom (see Cook's Note). Save the removed crumbs for another use.
Microwave the butter and a large pinch of salt in a small microwave-safe bowl until melted, about 1 minute. Brush the bread boxes inside and out with the melted butter. Put 1 slice of cheese at the bottom of each bread box. Put the boxes open-side up on a baking sheet and bake until the cheese melts and the inside of the bread is lightly browned and crispy in parts, about 10 minutes.
Meanwhile, heat 1 tablespoon of the oil in a large skillet over medium-high heat. Fill a small liquid measuring cup with 1/4 cup water. When the oil starts to shimmer, add the onion and a large pinch of salt. Cook, stirring, until the onions are tender and lightly browned, 6 to 8 minutes. If the pan ever gets too hot, stir in 1 tablespoon of water and use a wooden spoon or heat-proof rubber spatula to scrape up any browned bits from the bottom of the pan. Add another tablespoon of oil and the red and green bell peppers to the skillet. Season with a large pinch of salt and cook, stirring, until tender and lightly browned, 6 to 8 minutes. Add the remaining tablespoon of oil and the roast beef and cook, stirring, until the roast beef is just warmed through, about 2 minutes. Turn off the heat and stir in the cheese sauce.
Evenly divide the meat mixture among the bread boxes and top each with 2 more slices of cheese. Return to the oven and bake until the cheese melts, about 5 minutes. Serve whole or cut in half.
Don't worry if you accidentally make a hole at the bottom of the bread when carving out the boxes--just fill the hole with a loose piece of bread.
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