For the spice mix: In a small bowl, combine the basil, garlic powder, paprika, sea salt, cumin and oregano. Mix until well blended.
For the stew: Combine the flour with 1 tablespoon of the spice mix. Dredge the beef in the seasoned flour mixture, shaking off any excess flour.
Heat 2 tablespoons of the grapeseed oil in a large saute pan over medium-high heat. In two batches, sear the dredged beef on all sides, adding the remaining 2 tablespoons grapeseed oil before the second batch. Transfer the seared beef to a slow cooker.
Deglaze the saute pan with 1 cup of the beef broth, scraping up any browned bits left in pan; add to the slow cooker. Then, to the slow cooker, add the remaining beef broth and the carrots, celery, garlic, onions, tomatoes, potatoes, lentils, soy sauce, bouillon, pepper flakes, thyme and bay leaf.
Cover and cook on low for 12 hours or on high for 8 hours. Remove the bay leaf and thyme sprigs before serving.
Store the remaining spice mix in an airtight container at room temperature for up to three months.