Preheat oven to 450°F. Spray a large rimmed sheet pan or two pans with cooking spray.
Spread cabbage and onion on pan. Drizzle with oil, bacon drippings, salt, pepper, fennel seed and crushed red pepper flakes, tossing to coat. (You can use more or less oil to your taste and/or spray with cooking spray)
Place into the oven, roasting for 10 minutes. Stir, then arrange kielbasa slices on top.
Lower oven temp to 425°F and return to oven cooking for 25-35 minutes or until cabbage is lightly charred and tender, stirring halfway through. Broil at the end of cooking to brown further, if desired.
Serve immediately garnished with parsley and crumbled bacon.