Bring about 4 cups of water to a boil and throw in the cubed potatoes for 5 minutes. Drain the potatoes thoroughly. Put the cooked potatoes, the bell peppers, mushrooms, garlic, olive oil, and rosemary into a bowl and mix until the vegetables are well covered with the oil and rosemary. Spread the vegetables onto a broiler pan, sprinkle generously with salt and pepper, and broil for 10-12 minutes, until vegetables are slightly crisped and browned at the edges. Stir once or twice to ensure even cooking.

Return roasted vegetables to the bowl and toss with vinegar. Serve hot or at room temperature.