In a small bowl, whisk vinegar, mustard, salt and pepper until combined. Gradually whisk in olive oil until thickened.
Place Brussels sprouts and scallions on a large baking sheet and toss with 2 tbsp of the dressing. Roast at 400° for 22 minutes, turning once, until tender. Cool slightly.
In a large bowl, combine spinach, grapes and sunflower seeds. Toss with 4 tbsp of the remaining dressing. Add Brussels sprouts and scallions.
Top with cheese. Serve with remaining dressing and, if using, additional salt and pepper.
