Toss carrots, parsnips, beet, ½ teaspoon sea salt, thyme sprigs, and garlic together with two tablespoons olive oil on a baking sheet to coat well – use a little more oil if needed. Don’t overcrowd the veggies – leave a little space around the pieces so they don’t steam. Tip: If you have a smaller oven or baking sheet, it’s better to roast veggies in batches as needed. If you go this route simply re-warm all the veggies for a minute in the oven before serving if you’re craving them on the warmer side.
Roast veggies undisturbed in preheated oven 10-12 minutes until browned on the first side, then toss and roast another 10-12 minutes to finish. They should still have a bite to them – careful not to overcook. Tip: if after the first flip you’re finding that your veggies aren’t browning as they cook, increase oven temperature to between 450-475F and reduce baking time if needed.
Meanwhile, whisk the balsamic, remaining sea salt, chopped herbs and mustard seeds together in a small bowl. Pour the remaining olive oil in a thin stream while whisking to combine. Set aside.
To assemble: Discard thyme stems and arrange warm, roasted veggies on a serving platter with the spinach leaves. Drizzle with herb vinaigrette. Crumble goat cheese over top and garnish with dried cranberries. Serve immediately.