Begin by toasting your barley grains in a large stockpot. Place the stockpot on the stove and turn the heat to medium. When hot, add the hulled barley to the skillet and toast the grains until fragrant, stirring frequently. About 3-5 minutes. Set aside.
Bring 4 cups of water to a boil in a medium-sized sauce pan. When boiling, add the toasted barley and reduce to a low simmer. Allow the barley to cook, covered, until densely chewy. About 45 minutes. Set aside.
While the barley is cooking, add the cremini mushrooms and ¼ cup of water to the large stockpot. Bring to a simmer and cook until the water has cooked down. Then add 1 tbsp of olive oil and sauté the mushrooms until they begin to brown. Set aside.
Add the onions, 1 tsp of salt and 1 tbsp of olive oil to the stockpot. Reduce the heat to low and cover. Allow the onions to sweat for 10 minutes. At this stage, the onions should be becoming soft and translucent.
Remove the cover from the onions and continue to cook for another 20 minutes on low heat, stirring occasionally. The onions should begin to brown slowly. After 20 minutes, turn the heat up to medium-low and allow the onions to fully brown for another 10 minutes. They should remain soft (you’re not trying to crisp the onions but allow the natural sugars in the onions to caramelize).
While the onions are cooking, combine the tamari, maple syrup, balsamic vinegar, paprika and pepper in a small bowl. Stir to combine and set aside.
When the onions are a rich brown, add the garlic and cook for 1-2 minutes more.
Add the dried rosemary and parsley. Stir to combine and cook until fragrant. About 1 minute.
Add the tomato paste and stir to combine. Allow the tomato paste to toast in the pan until it is a dark, rich red. About 2-3minutes.
Add the tamari mixture to the onions and stir to combine. The liquid should deglaze the bottom of the pan. Immediately add the potatoes, mushrooms, carrots and parsnip. Stir to coat the vegetables with the sauce.
Add the water, diced tomatoes and bay leaves. Bring to a low simmer and allow the broth to cook until the vegetables are easily pierced with a fork. About 25-30 minutes. Remove the bay leaves.
If the stew is too thin, combine the cornstarch and water in a small bowl and whisk until no clumps remain. Add the cornstarch slurry to the stew and continue cooking for ~5 minutes until the broth has thickened.
Serve the stew by filling the bottom of each serving bowl with the cooked barley. Spoon the stew over the barley and season with salt & pepper to taste.