Adjust oven rack to upper-middle position and heat oven to 500 degrees. Quarter cabbage through core and cut each quarter into 2 wedges, leaving core intact. Arrange wedges, 1 flat side down, on rimmed baking sheet. Brush 1 ½ tablespoons oil on exposed cut sides of wedges and sprinkle with ½ teaspoon salt.
While cabbage roasts, prepare topping. Cook bacon in 10-inch skillet over medium heat, stirring frequently, until bacon is crisp and fat is rendered, about 5 minutes. Transfer bacon to paper towel-lined plate to drain, and discard fat or reserve for other use. Add vinegar to hot skillet and stir, scraping up browned bits. Whisk in cream and mustard until smooth.
Remove foil (be careful of escaping steam) and continue to roast until cabbage wedges begin to brown on underside, 5 to 10 minutes. Using tongs and thin metal spatula, flip each wedge. Roast until well browned and some leaves have crisped, 5 to 10 minutes. Drizzle half of mustard sauce over platter. Arrange roasted cabbage over sauce on platter. Drizzle with remaining mustard sauce, sprinkle with parsley and bacon, and serve.
