Preheat oven to 425 degrees.
Add apples, scallions, and half of the cabbage to a baking sheet. Toss with olive oil and 1 tsp. kosher salt. Roast until cabbage is brown and ends are starting to crisp, 25-30 minutes. Toss halfway through.
To make dressing, add shallot, lemon juice, mustard, olive oil, and a punch of salt to a jar. Shake for 30 seconds to combine.
Add the other half of cabbage to a large bowl. Sprinkle with 1/2 tsp. salt and 1/4 cup of the dressing. Toss and gently massage cabbage with hands until slightly wilted. Let sit while the other ingredients finish roasting.
Roughly chop roasted scallions. Add roasted cabbage, apples, and scallions to bowl with cabbage. Shake lemon mustard vinaigrette for a few seconds to combine if needed and add remaining 1/4 cup to bowl. Toss to combine. Season with salt accordingly and top with sunflower seeds.