Preheat oven to 400 degrees. In the meantime, add mustard, jam, and ½ teaspoon olive oil to a jar with a tight-fitting lid and shake until well mixed; set aside. 

Lightly grease a large rimmed baking sheet. If necessary, remove any bruised or dirty outer cabbage leaves, trim the bottom of the core, and split the head in half. Cut the half you’re using for this recipe in ½”-thick slices and place on the baking sheet. Drizzle the tops of the cabbage slices with olive oil and sprinkle with salt and pepper. 

If your sausage is in a loop, you can carefully cut slices every ½” or so, being careful not to cut all the way through. Think hassleback style. (This is optional!) Nestle the sausage on the baking sheet.

Using a basting brush, brush the tops of all the sausage and slices of cabbage with the mustard mixture, being sure to reserve about 1 tablespoon. Place in the oven and roast for about 35 minutes, until the sausage is heated through and the cabbage is fork tender and lightly browned in bits.

While the sausage and cabbage cook, add the sour cream, lemon zest, lemon juice and 2 tablespoons of water to the jar with the remaining tablespoon of mustard-jam mixture. Shake until well combined: this is your drizzle sauce.

Once finished cooking, remove the sausage and cabbage from the oven, plate, and drizzle each portion with sauce, as desired. Top with chives, if using.