Place a rimmed sheet pan in the oven and preheat to 375 degrees F.
Mix together the mushrooms, onions, peppers and cauliflower in a large bowl, then lightly drizzle with olive oil and sprinkle with some salt. Add the white vinegar and toss to combine.
Pour vegetables and vinegar into the preheated sheet pan. Bake, rotating halfway through, until vegetables are tender and beginning to brown and the vinegar has evaporated, 15 to 20 minutes. Garnish with parsley.