Mince the garlic and dice the onion. Heat grapeseed oil in a large, heavy-bottomed pot or Dutch oven over medium-high heat. Add garlic and sauté until light brown, 1–2 minutes. Add onions and a pinch of salt. Stir and sweat the onions, 3–4 minutes.
Peel and chop the carrots; chop the celery. Stir in the celery, carrots, and tomato paste. Let the tomato paste brown in the pan a few minutes. Stir in cumin and coriander, followed by thyme and chopped tomatoes. Reduce heat to low and cook 5 minutes; meanwhile, peel and dice the sweet potato. Stir in the sweet potato, and cook 5 more minutes.
Purée chicken stock and cashews in a blender. Add to the vegetables and stir. Increase heat to medium-high and bring stew to a simmer. Cook uncovered until liquid is reduced by half, 35–40 minutes.
When stew is reduced, core and dice the apples and stir them in. Ladle into bowls and serve hot.