On a sheet pan combine onion, cauliflower, potatoes, apples, oil and all the spices and toss to combine and then spread out evenly.
Bake for 25 minutes, stirring halfway through.
Remove and move all of the vegetables towards the middle of the sheet pan, creating a small pile. Place kielbasa on top and drizzle the sausage with a bit of avocado oil and place back in the oven for 15 minutes.
Remove from oven and place 1-½ cups of sauerkraut on one side of the sheet tray and continue to bake until the sauerkraut is just slightly warmed, about 3 minutes.
Remove and let sit until the kielbasa is cool enough to handle, about 2 minutes. Slice kielbasa into ¼” slices on the bias and divide sausage and veggies into four portions. Serve with whole grain mustard.
If Whole30 check to make sure your kielbasa is compliant.